Preheat oven to 350°F. Butter a 10-inch springform baking pan and dust with cocoa powder. Pour beer into a glass or cup and let rest for 5 minutes while you prepare ingredients.

Pour the Guinness into a large saucepan, adding the butter cut into slices. Heat until butter is melted. Once melted, whisk in cocoa powder and sugar, whisking gently until incorporated and dissolved.

Beat the sour cream, eggs and vanilla in a small bowl until creamy and smooth. Pour into the buttery beer batter, whisking until smooth. Finally, whisk in flour and baking soda. DO NOT OVERMIX. Mix until combined. (A few lumps are okay.)

Pour batter into greased and lined springform pan. (If pan is really full, place on half sheet pan to protect the oven from spillage). Bake on center rack in middle of oven for 45 minutes to 1 hour. Remove once a toothpick inserted comes out without crumbs.

Allow to cool completely in the springform pan on a cooling rack. Once cooled, cake may be frosted or may be wrapped and frozen until ready to frost.

Add softened butter to bowl of a stand mixer (or hand mixer) fitted with a paddle or whisk attachment. Beat on medium-high speed for 2-3 minutes, until light and fluffy. With mixer on low or stir, slowly add powdered sugar. Beat on low speed until mixed into butter, then increase speed to medium-high and beat for 1 minute, scraping down the sides as needed.

Add in sea salt, vanilla and Irish cream liqueur (substitute with heavy cream, milk or half-and-half and add a teaspoon of espresso powder), beating on low speed until combined. Increase speed to high and beat until light and fluffy, about 3-4 minutes. Stop and scrape down sides once or twice.

Frost cooled cake. If not eating right away, store covered in the fridge until ready to eat and bring to room temperature before serving.