Preheat the oven to 350°F (175°C). Grease and line two 9-inch round cake pans with parchment paper.

In a saucepan over medium heat, combine Guinness and butter. Heat until the butter melts completely. Remove from heat and whisk in cocoa powder, granulated sugar, and brown sugar until smooth. Let cool slightly.

In a large mixing bowl, whisk together eggs, vanilla extract, and sour cream. Slowly mix in the cooled Guinness mixture until fully combined.

In another bowl, whisk together flour, baking soda, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, mixing just until combined.

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. Let cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

For the buttercream, beat softened Irish butter in a mixing bowl until light and fluffy. Gradually add powdered sugar, alternating with heavy cream, and beat until smooth. Mix in vanilla extract and Irish whiskey (if using) until fully incorporated.

Assemble the cake by spreading a layer of buttercream on the first cake layer. Top with the second layer and frost the top and sides of the cake. Smooth with an offset spatula.

Slice and serve this decadent cake at room temperature. Enjoy!