1. Grease a 9×13 cake pan and preheat oven to 350°F (176°C).
2. In a large mixing bowl, beat the butter and sugars together until light in color and fluffy, 3-4 minutes.
3. Add the sour cream and vanilla extract and mix until combined.
4. Add the eggs in two batched and mix until incorporated. Scrape down the sides of the bowl as needed to make sure everything is combined.
5. Combine flour, cocoa powder, baking powder and salt in a medium bowl.
6. Add half of the flour mixture to the batter and mix until combined.
7. Add the Guinness to the batter and mix until combined. Scrape down the sides of the bowl as needed to ensure everything is being combined.
8. Add the remaining flour mixture and mix until smooth.
9. Pour the cake batter evenly into the prepared cake pan and bake 32-35 minutes, or until a toothpick inserted in the center comes out with a few crumbs. Remove the cake from oven and poke holes all over the top of the cake.
10. Place sweetened condensed milk, chocolate chips and Guinness in a microwave safe bowl. Microwave for about 30 seconds to 1 minute, until warm.
11. Whisk chocolate and milk until smooth, microwaving more, if needed to melt the chocolate.
12. Pour chocolate mixture over the cake while still warm and spread to fill in holes.
13. Set cake aside to cool, about an hour, then refrigerate until completely cooled.
14. To make whipped topping, whip heavy whipping cream and Guinness until it begins to thicken.
15. Add the powdered sugar, cocoa and vanilla extract and whip until stiff peaks form.
16. Spread whipped topping evenly over cooled cake.
17. Sprinkle mini chocolate chips over cake and drizzle with chocolate sauce. Refrigerate until ready to serve.