Chocolate tart crust (allow the crust to cool while preparing the ganache filling and whipped cream):
Pre-heat oven to 350 degrees F. Place the flour, cocoa powder and salt in a food processor and blend to mix. Add the butter and mix until the dough is crumbly and is composed of large (garbanzo-bean sized) crumbs. Add the yolk and water and run the machine is small pulses. Do so until the dough just comes together, so as to not overwork it. Flip dough onto counter, knead until it comes together, then gather into a square. Wrap in plastic wrap and refrigerate for 20 minutes or freeze for 5 minutes. When slightly cooled, bring dough out of the fridge and, on a floured surface with a floured pin, roll the dough out into a rough circle. Tart dough is often brittle, so do not worry about cracks or splits. Once rolled out, transfer dough to a 10-inch tart pan and press into the ridged sides. Use extra dough to cover any imperfections. Using a fork, poke holes in the bottom and place in the oven for 10-15 minutes. Take out and let cool.
Chocolate ganache filling: In a small mixing bowl, combine 1 Tbsp heavy cream and egg yolks until mixture comes together consistently. Do not over mix. Set aside. Pour remaining cream into a saucepan and bring to a full boil, stirring to avoid a skin coating or burning of the cream. Once achieved, pour into a heatproof bowl over the chocolate chips. Wait 30 seconds until melted, then slowly whisk to bring together the chocolate and the cream . Next, incorporate the egg mixture and butter until smooth. Finally, add the lemon flavoring extract and stir. Pour ganache into crust. Let the filling set by placing it in the fridge for 10 minutes, then bring down to room temperature on counter. Serve at room temperature with lemon-slice garnish and sour cream whipped cream.
Sour cream whipped cream: Bring all ingredients together in a standing mixer bowl and whip until stiff peaks arise. Serve immediately or store in fridge for up to a day.