Preheat the oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan or two round cake pans.
In a large mixing bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, cinnamon, cayenne pepper (if using), and salt until well combined.
In a separate bowl, beat the eggs, milk, vegetable oil, and vanilla extract until smooth.
Gradually add the wet ingredients to the dry ingredients, mixing until the batter is smooth and no lumps remain.
Carefully stir in the boiling water or brewed coffee. The batter will be thin, which is normal.
If desired, fold in the melted Mexican chocolate or dark chocolate for extra richness.
Pour the batter evenly into the prepared pan(s).
Bake for 35–40 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely before frosting or serving.