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

In a large mixing bowl, combine granulated sugar, light brown sugar, flour, cocoa powder, espresso powder, baking soda, baking powder, and salt.

In another bowl, whisk together the eggs, sour cream, Guinness, milk, vegetable oil, and vanilla until smooth.

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

Pour the batter into the Bundt pan and bake for 50 minutes or until a toothpick comes out clean.

Let the cake cool in the pan for 15 minutes, then transfer to a wire rack to cool completely.

Beat room temperature unsalted butter until fluffy, then gradually add the confectioners' sugar.

Mix in Baileys Irish Cream, whole milk, vanilla extract, and salt until creamy.

Fold in melted white chocolate until well blended.

Frost the cooled cake with the Baileys buttercream.

Combine sugar and water in a saucepan over medium heat, stirring until sugar dissolves.

Cook without stirring until it reaches a golden hue, then remove from heat.

Whisk in unsalted butter, whipped cream, Irish whiskey, vanilla, and salt, then cool to room temperature.

Drizzle over the frosted cake before serving.