Preheat oven at 350 degrees.
Shift all dry ingredients (excluding sugar) together into a large bowl. Set aside. Dry ingredients include flour, baking powder, salt, cocoa powder, and baking soda.
Using a mixer, cream together sugar and butter until blended well together. Scrap the sides of the bowl to ensure everything is mixed well together.
In a medium to large size bowl place boiling water, coffee, and semi sweet chocolate. Stir until chocolate has melted, and the ingredients are mixed well. Let the melted chocolate mixture cool down.
Once the chocolate mixture has cooled down, add the wet ingredients to the melted chocolate. These wet ingredients include the Mexican vanilla, eggs, buttermilk, and sour cream. Mix well.
All the wet ingredients to the creamed sugar and butter. Mix well.
Next, add the sifted dry ingredients to the mixing bowl, mix just until well combined. Be careful not to over mix the batter.
Now grease your cake pan. You do this by mixing the melted butter and one additional tablespoon of cocoa powder together. Brush this butter and cocoa mixture all over the cake pan. If the mixture gets thick while brushing the cake pan, heat the mixture in the microwave for a few seconds to loosen up the mixture.
Now add the cake batter to the cake pan.
Place the cake pan in the oven. Bake the cake for 45 to 50 minutes or until a toothpick inserted comes out clean.
Remove cake from oven, and let it cool completely.