Instructions

Make the chocolate pate sucree:

1. Place the flour, cocoa powder, icing sugar, and salt in
   the bowl of a food processor. Pulse a few times to combine.
2. Scatter the butter over the top of the flour mixture. Pulse several times until the butter is about the size of peas.
3. Whisk together the egg and vanilla. Add the egg mixture in three stages, pulsing after each addition. Once all the egg is added, pulse in 10-second increments until the dough forms clumps but is not completely smooth. Once the dough reaches this stage, remove half of the dough and place on a piece of parchment. (You only need half the dough for this recipe, so you can wrap and freeze the remainder for the future or try one of my ideas for extra tart dough!)
4. Place a piece of plastic wrap on top of the dough. Use your hands to press the dough into a round, then roll it out into a roughly 12-inch (30-cm) circle about ¼-inch (6-mm) thick. Lift and replace the top piece of plastic occasionally to avoid creases in the dough.
5. Slide the sheet of dough onto a baking sheet (still sandwiched between the parchment and plastic). Refrigerate until cold, at least 1 hour. (This is a good time to make the chocolate pastry cream base.)
6. Once chilled, remove the unbaked crusts from the fridge and let it stand for about 5 to 10 minutes, just long enough to make it pliable. Lightly grease a 9-inch tart tin. Turn the dough into the tart tin. Gently lift the edges and press the dough into the bottom and sides of the tin. Don’t worry if it cracks or breaks here and there; the dough is forgiving and you can gently press it back together.
7. Trim the edge of the dough even with the top of the tart tin (you can gently roll a rolling pin over the top or use a small paring knife). If there are any tears or cracks, use some of the extra dough to patch it up; reserve the extra dough for repairing any cracks that might appear after baking. Pierce the bottom of the dough all over with a fork, then wrap in plastic and freeze for at least 30 minutes.
8. Preheat the oven to 375°F (190°C) with a rack in the middle. Remove the chilled tart dough from the freezer. Lightly grease the dull side of a piece of foil and fit it firmly over the chilled tart dough. Place the tart tin on a baking sheet and bake for 20 minutes. Remove the foil (if the crust has risen at all, press it down gently with the back of a spoon). If any cracks have formed, use some of the reserved dough to patch them. Bake the crust or another 5 to 10 minutes, or until firm and dry. Transfer to a wire rack and allow to cool completely before filling.

Make the chocolate creme legere:

1. Place a strainer over a medium heat-safe bowl or container.
2. Off heat, combine the milk, 20g of the sugar and a pinch of salt in a medium saucepan.
3. In a medium bowl, whisk together the remaining 45g sugar, cornstarch, and cocoa powder. Pour in about 2 tbsp of the milk mixture and whisk to form a smooth paste. Add the egg yolks and whole egg and whisk until smooth.
4. Heat the milk over medium heat until steaming. Remove from the heat. Pour the milk in a slow, steady stream into the egg yolk mixture, whisking constantly. Scrape the custard mixture back into the saucepan and return to medium heat. Cook, whisking constantly, until the mixture thickens and large bubbles appear on the surface. Once the bubbles appear, turn the heat down to medium-low and continue whisking on the heat for 2 minutes.
5. Remove the custard from the heat and whisk in the vanilla extract, butter, and chocolate. Strain the pastry cream into the prepared bowl. Press a piece of plastic wrap against the surface of the pastry cream and cool to room temperature, then refrigerate until set (at least two hours, or up to 3 days). (This is a good time to bake and cool the tart crust and make the cherry ganache.)
6. When ready to fill the tart, whip the heavy cream, sour cream, and icing sugar to medium peaks. Whisk the pastry cream to loosen, then use a flexible spatula to gently fold the whipped cream into the pastry cream in three additions. Fold just until homogenous and no streaks of cream remain. Use immediately.

Make the cherry ganache:

1. Prepare the ganache once the tart case is baked and cooled. In a small saucepan over medium heat, melt the butter. Add the cherries and saute, stirring frequently, until soft and the juices have released, about 5 minutes. Remove from heat and stir in the liqueur, if using. Let cool for 10 minutes, then use an immersion blender or small food processor to blitz until smooth.
2. In a medium microwave-safe bowl, combine the chocolate and cream. Heat in 20-second intervals, stirring well after each burst, until the chocolate is 80% melted. Continue stirring until the chocolate is completely melted and the ganache is emulsified. Stir in the cherry puree (you can use an immersion blender if you want the ganache extra smooth). Pour ganache into the cooled tart case and tilt to cover the entire bottom. Chill in the refrigerator to set (this should happen within 10 minutes as the layer is thin).

To finish:

1. Spread the chocolate creme legere over the set ganache. (You may have a little extra; enjoy as a baker's treat!) Press a piece of plastic over the top and chill for at least an hour to set.
2. Right before serving, decorate with fresh cherries. (If not planning to eat the entire tart that day, you can brush the cherries with some apricot jam to keep them from drying out and discoloring.) Store leftovers in an airtight container for up to 3 days.