Preheat the oven to 350°F. Grease a 9x13 inch baking pan.

Beat the butter, sugar, and brown sugar together until fluffy. Add sour cream and vanilla, mix well.

Add eggs one at a time, mixing after each addition. In a separate bowl, whisk together flour, cocoa, baking powder, and salt.

Alternate adding flour mixture and Guinness to the batter, starting and ending with flour mixture. Mix until just combined.

Pour batter into the prepared pan and bake for 32-35 minutes. Allow to cool slightly.

While cake cools, microwave condensed milk, chocolate chips, and Guinness for 1 minute, stirring until smooth. Poke holes in the cake and pour the warm chocolate mixture over it.

Refrigerate until completely cooled, about 1 hour.

Make whipped cream by whipping heavy cream, Guinness, powdered sugar, cocoa, and vanilla until stiff peaks form.

Spread whipped cream over the cooled cake, sprinkle mini chocolate chips, and drizzle with chocolate sauce.