Preheat oven to 350°F. Grease one 8-inch round cake pan. Set aside.

In a medium bowl, sift together flour, cocoa powder, baking powder, baking soda and salt. Set aside.

In a medium saucepan over medium-high heat, melt butter and chopped chocolate.

Once melted, stir in sugar. Remove from heat and let cool for 5 minutes.

Whisk in Guinness and vanilla extract until smooth. Quickly beat in egg until combined. Whisk in dry ingredients until smooth.

Pour batter into prepared pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.

Let cake cool in pan until completely cool, then transfer to cake plate and frost.

Cream butter in the bowl of a stand mixer on high until light and fluffy. Add Baileys Irish Cream and beat until combined.

Add powdered sugar 1 cup at a time, mixing thoroughly after each addition. After all sugar has been added, give it a taste and decide if you would like a little more Bailey's Irish Cream. If yes, add 1 teaspoon at a time until desired flavor is achieved.

Transfer frosting to a piping bag, and pipe along the top of the cake. Serve immediately or store in an airtight container for up to 3 days.