Prepare the tart crust. Pulse flour, cocoa powder, sugar, and salt in a food processor until well combined. Add the cold butter and pulse until the mixture resembles coarse meal. Add egg and process until the dough holds together, about 30 seconds. Shape the dough into a disk and cover with plastic wrap. Refrigerate for 30 minutes.
Remove the dough from the refrigerator and roll 1/8-inch thick. Transfer to the tart pan. Press the dough into the bottom and against the sides of the pan. Prick the dough all over with a fork. Cover with plastic wrap and freeze the unbaked tart crust for 1 hour.
Preheat oven to 375°F. Crumple a piece of parchment paper and set on top of the tart crust. Weight the parchment paper with dried beans or pie weights. Bake for 12-14 minutes until the tart looks dry. Remove parchment paper and beans and continue baking for 10 minutes. Remove from oven and transfer to a cooling rack.
In a medium saucepan, heat 1 cup of cream until boiling. Lower the heat and add the Mexican chocolate, stir until dissolved. Remove the mixture from the heat and add the rest of the chocolate. Stir until all the chocolate is melted and smooth. Add chili powder. Cool mixture to room temperature. Reserve ½ cup of chocolate mixture in a heat-resistant measuring cup.
Whip the chilled cream until stiff peaks. Fold into the cream the rest of the chocolate mixture until well combined. Add chocolate cream into the prepared tart crust. Refrigerate for 1 hour.
Remove tart from the refrigerator. In the microwave, heat the reserved chocolate mixture for 30 seconds until smooth and the consistency is pourable. Pour the chocolate over the tart.
Decorate with chopped Mexican chocolate, chocolate chips, and chili powder.