Begin by reducing the Guinness. Pour the beer into a large saucepan and bring it to a boil over medium-high heat. Once boiling, reduce the heat and simmer until the liquid reduces to about ¾ cup, roughly 30 minutes. Use ½ cup for the cake batter and reserve the rest for the frosting.

Preheat the oven to 350°F (177°C). Grease two 9-inch cake pans, line with parchment paper, and grease the parchment.

In a large bowl, whisk together flour, cocoa powder, sugar, baking soda, baking powder, salt, and espresso powder.

In another bowl, beat the oil, sour cream, eggs, and vanilla until combined. Mix in the buttermilk. Add the wet ingredients to the dry ingredients and stir briefly. Pour in ½ cup of the hot reduced Guinness and mix until smooth.

Divide the batter evenly between the prepared pans. Bake for 25–28 minutes, or until a toothpick inserted in the center comes out clean.

Let the cakes cool completely in their pans on a wire rack.

For the frosting, beat cream cheese and butter together until smooth and creamy. Add powdered sugar, cocoa powder, 3 tablespoons cooled reduced Guinness, and a pinch of salt. Beat until smooth and fluffy.

Level the cake layers if needed. Place one layer on a serving plate and spread about 1 cup of frosting on top. Add the second layer and frost the top and sides of the cake.

Refrigerate the frosted cake for 30–60 minutes before slicing to help it set.