Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans.

In a large mixing bowl, whisk together the Guinness beer and melted butter until combined.

Add granulated sugar, buttermilk, eggs, and vanilla extract, mixing until smooth.

In another bowl, sift together the flour, cocoa powder, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Divide the batter evenly between the prepared cake pans and smooth the tops.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted in the center comes out clean.

Once baked, allow the cakes to cool in the pans for 10 minutes before transferring them to wire racks to cool completely.

In a medium bowl, beat the softened cream cheese until smooth.

Gradually add the heavy cream, powdered sugar, and vanilla extract, beating until the mixture is fluffy and well combined.

Once the cakes are completely cooled, place one layer on a serving plate.

Spread a layer of cream cheese frosting over the top of the first cake layer.

Place the second cake layer on top and frost the top and sides of the cake with the remaining frosting.

Decorate with chocolate shavings or cocoa powder if desired.