Preheat your oven to 325°F (163°C) and generously spray a Bundt pan with nonstick baking spray.

In a large mixing bowl, combine granulated sugar, light brown sugar, all-purpose flour, unsweetened cocoa powder, espresso powder, baking soda, baking powder, and salt. Whisk until blended.

In a separate bowl, whisk together eggs, sour cream, Guinness beer, milk, vegetable oil, and vanilla extract until smooth.

Pour the wet ingredients into the dry ingredients and gently fold together until just combined.

Carefully pour the batter into the prepared Bundt pan and smooth the top.

Bake for approximately 50 minutes, checking with a toothpick for doneness.

Once baked, cool the cake in the pan for 15 minutes, then invert onto a cooling rack.

Beat unsalted butter in a bowl until creamy, then gradually mix in confectioners' sugar. Add Baileys Irish cream, whole milk, vanilla extract, and salt, mixing until fluffy.

Frost the cooled cake with Baileys buttercream and drizzle with salted caramel if desired.