For the Cake:

1. Preheat the oven to 350°F.
2. In a large mixing bowl, combine the sugar, vegetable oil, eggs and vanilla extract. Beat on medium speed until well incorporated. Next mix in the sour cream and beat until well combined.
3. In a separate bowl, sift together the dry ingredients. To the batter, alternate adding half the dry ingredients followed by half of the Guinness and mix just until the flour starts to incorporate. Repeat with remaining ingredients. Use a spatula to scrape down the sides of the bowl and stir the batter from the bottom to the top to ensure it’s well mixed.
4. Pour the batter into a 9×13-inch pan and bake at 350°F for 28-30 minutes. Test the cake for doneness by inserting a toothpick into the center of the cake. If the toothpick comes out clean, the cake is done. Remove from the oven and allow to cool completely.

For the Frosting:

1. Remove the butter from the refrigerator and microwave for 10 seconds. Cut the butter into 1-inch blocks. Beat the butter for 2-4 minutes until it is softened and silky.
2. Add 2 cups of powdered sugar and the vanilla extract and beat until well combined.
3. Next, add the remaining powdered sugar 2 cups at a time, followed by the Baileys. Start beating slowly until the ingredients are incorporated and then increase the speed to high and beat for another minute or two until the frosting is light and fluffy,
4. Spread frosting over cooled cake and then dust with cocoa powder, sprinkles or chocolate ganache. Keep in an airtight container.