For the Cake:

1. Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans (or line with parchment paper).
2. In a large bowl, sift together the flour, sugar, cocoa powder, baking soda, baking powder, salt, and instant coffee (if using).
3. In a separate bowl, whisk together the eggs, buttermilk, vegetable oil, and vanilla extract.
4. Gradually add the wet ingredients to the dry ingredients, mixing well until the batter is smooth.
5. Stir in the sour cream until fully incorporated. The batter will be quite thin, but that’s normal.
6. Carefully pour in the boiling water and mix until the batter is smooth (the batter will remain thin).
7. Divide the batter evenly between the prepared cake pans.
8. Bake for 30-35 minutes, or until a toothpick inserted into the center of the cake comes out clean.
9. Allow the cakes to cool in the pans for 10 minutes, then transfer to wire racks to cool completely.

For the Frosting (Optional):

1. In a large bowl, beat the softened butter until creamy and smooth.
2. Gradually add in the powdered sugar and cocoa powder, beating until fully incorporated.
3. Add vanilla extract and milk, one tablespoon at a time, until you reach the desired consistency for spreading.
4. Frost the cooled cakes with the chocolate frosting.