Rich Mexican chocolate cake with hints of cinnamon and chile is an indulgent treat. This family celebration dessert is a decadent, complex, deeply flavored chocolate experience.
Preheat the oven to 350°F. Grease a 9-inch round cake pan and line with parchment paper to prevent sticking.
In a microwave-safe bowl, melt the Mexican chocolate together with the butter in 30-second bursts, stirring until smooth. Allow to cool slightly.
In a large bowl, whisk together flour, cocoa powder, baking soda, baking powder, salt, cinnamon, and cayenne pepper for even distribution of leavening and spices.
In another bowl, beat eggs and sugar until pale and thick, about 2 minutes on medium-high speed. Add the vanilla extract, then mix in the cooled chocolate-butter mixture.
Add the dry ingredients to the wet mixture in thirds, alternating with sour cream and milk, mixing gently on low speed to avoid overmixing and preserve cake tenderness.
Pour batter into the prepared pan and smooth the top. Bake for 38–42 minutes, checking for doneness with a toothpick; it should come out with moist crumbs, not wet batter.
Cool in the pan for 10 minutes, then run a knife around the edge and transfer the cake to a wire rack to cool fully before serving for best flavor and structure.