Preheat your oven to 350°F (175°C). This ensures that your tart bakes evenly.
In a large mixing bowl, combine the granulated sugar, flour (or buckwheat flour), cocoa powder, and salt. Whisk them together until well blended. Pour in the melted butter and mix until the dough resembles wet sand. Press the mixture into the bottom and up the sides of your tart pan evenly. Poke a few holes in the bottom with a fork to prevent bubbling.
Place the tart crust in the preheated oven and bake for about 10-12 minutes. Once baked, remove it from the oven and allow it to cool while you prepare the filling.
In a medium saucepan, combine the whipping cream, chocolate chips, peppermint extract, and vanilla extract. Heat over medium-low, stirring constantly until the chocolate is fully melted and the mixture is smooth. Remove from heat.
Allow the chocolate mixture to cool slightly before whisking in the beaten eggs. Mix until fully combined.
Pour the chocolate filling into the cooled tart crust. Spread it evenly and smooth the top with a spatula.
Return the tart to the oven and bake for an additional 20-25 minutes. The filling should be set but slightly jiggly in the center.
Remove the tart from the oven and let it cool at room temperature for about 30 minutes. Once cooled, refrigerate for at least 2 hours to let it set completely.