Preheat your oven to 350°F (175°C). Grease two 8-inch round cake pans and line the bottoms with parchment paper.

In a medium saucepan, heat the Guinness stout and cubed butter over medium heat, stirring until the butter is completely melted. Whisk in the cocoa powder and granulated sugar until smooth and glossy. Allow to cool for a few minutes.

In a large mixing bowl, whisk together the sour cream, eggs, and vanilla extract. Gradually whisk in the cooled Guinness-chocolate mixture until well combined.

Sift together the flour and baking soda directly into the wet mixture. Gently mix until combined, being careful not to overmix.

Divide the batter evenly between the prepared pans. Bake for 35–40 minutes, or until a toothpick inserted comes out clean. Let cool in pans for 10 minutes before transferring to a wire rack.

In a mixing bowl, beat the unsalted butter until light and fluffy. Gradually add powdered sugar, one cup at a time, mixing well. Then add the Baileys and a pinch of salt, beating until smooth and creamy.

Once the cake layers are fully cooled, spread Baileys buttercream between them. Use remaining frosting to decorate the top and sides, adding sprinkles if desired.