Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Whisk together until well blended.
In another bowl, mix the buttermilk, vegetable oil, eggs, and vanilla extract until smooth. Gradually add the wet ingredients to the dry ingredients, mixing until just combined.
Stir in the Bailey's Irish Cream and hot water until the batter is smooth.
Divide the batter evenly between the prepared cake pans. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.
For the frosting, beat the softened butter in a large bowl until creamy. Gradually add the powdered sugar, mixing until combined. Add the Bailey's Irish Cream and vanilla extract, mixing until smooth. If desired, add cocoa powder for a chocolate frosting.
Once the cakes are completely cool, place one layer on a serving plate and spread a layer of frosting on top. Place the second layer on top and frost the top and sides of the cake.
Serve immediately or refrigerate for up to 3 days.