Preheat your oven to 325°F (163°C) and prepare your 10-cup bundt pan with nonstick baking spray.

In a large mixing bowl, whisk together the granulated sugar, light brown sugar, all-purpose flour, unsweetened cocoa powder, espresso powder, baking soda, baking powder, and salt.

In another bowl, whisk the eggs, full-fat sour cream, Guinness, whole milk, vegetable oil, and vanilla extract until smooth.

Pour the wet mixture into the bowl of dry ingredients, using a spatula to gently stir until just combined.

Transfer the batter into the prepared bundt pan and gently tap to release air bubbles.

Place the bundt pan in the oven and bake for about 50 minutes, or until a toothpick comes out clean.

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

Beat the unsalted butter until smooth, then gradually add confectioners' sugar and mix in Baileys, whole milk, vanilla extract, and salt to make the buttercream.

Frost the cooled cake generously with Baileys Buttercream, allowing some to cascade down the sides.

Optional: Make a salted caramel by cooking sugar and water until dissolved, stir in butter, then cream and whiskey.