In a large saucepan, bring the Guinness to a boil over medium-high heat. Once boiling, reduce to medium-low heat and allow to simmer until reduced down to about 3/4 cup (180g/ml), about 30 minutes. Begin making the cake batter once the Guinness is simmering. You’ll use 1/2 cup (120g/ml) of the hot reduced Guinness in the cake batter, and set aside the rest to cool for the frosting. You can do this step 2 to 3 days ahead of time if needed—see Note.

Preheat oven to 350°F (177°C). Grease two 9-inch cake pans, line with parchment paper rounds, then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans. (If it’s helpful, see this parchment paper rounds for cakes video & post.)

While the Guinness is simmering, whisk the flour, cocoa powder, sugar, baking soda, baking powder, salt, and espresso powder (if using) together in a large bowl. Set aside.

Using a handheld or stand mixer fitted with a whisk attachment (or you can use a whisk) mix the oil, sour cream, eggs, and vanilla together on medium-high speed until combined. Add the buttermilk and mix until combined. Pour the wet ingredients into the dry ingredients, give it a quick mix/whisk, then add 1/2 cup (120g/ml) of hot reduced Guinness. Mix or whisk on low speed until the batter is completely combined. Batter is thin. If it helps, you should have about 6 cups or 1250g of cake batter total.

Divide batter evenly between pans. Bake for 25–28 minutes or until a toothpick inserted in the center comes out clean. Baking times vary, so keep an eye on yours. The cakes are done when a toothpick inserted in the center comes out clean. (Note: Even if they’re completely done, the cooled cakes may slightly sink in the center. Cocoa powder is simply not as structurally strong as all-purpose flour and can’t hold up to all the moisture necessary to make a moist chocolate cake. It’s normal!)

Remove the cakes from the oven and set on a wire rack. Allow to cool completely in the pans.

With a handheld or stand mixer fitted with a paddle attachment, beat the cream cheese and butter together on medium speed until creamy and smooth, about 2 minutes. Add confectioners’ sugar, cocoa powder, 3 Tablespoons (45g/ml) cooled reduced Guinness, and a pinch of salt. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 full minute. Do not over-whip. Add a couple more Tablespoons of confectioners’ sugar or cocoa powder if frosting is too thin, or 1 more Tablespoon of cooled reduced Guinness if frosting is too thick. Taste. Add another pinch of salt if desired.

If cooled cakes are domed on top, use a large serrated knife to slice a thin layer off the tops to create a flat surface. This is called “leveling” the cakes. Discard or crumble over finished cake. Place 1 cake layer on your cake stand or serving plate. Using a large icing spatula or small offset spatula, evenly cover the top with about 1 cup (about 250g) of frosting. Top with 2nd layer and spread remaining frosting all over the top and sides. A bench scraper is helpful for smoothing out the sides. I usually go lighter on the sides of the cake, to have leftover frosting for piping around the top. (Piping is optional.) If desired, garnish top of cake with sprinkles.

Refrigerate uncovered cake for at least 30–60 minutes before slicing to help set the shape. After that, you can serve the cake or continue refrigerating for up to 4–6 hours before serving. Cake can be served at room temperature or chilled.

Cover leftover cake tightly and store in the refrigerator for 5 days. I like using a cake carrier for storing and transporting.