For the Cake: Position rack in middle of oven. Preheat oven to 350°F/180°C. Generously coat the inside of a 9-inch (23 cm) springform pan with nonstick spray. Sprinkle with 2 tablespoons of extra cocoa, rotating the pan to coat well with the cocoa; tap out any excess. Set aside.

Place beer and butter in a medium sized saucepan and heat over medium heat to melt butter. Whisk together until combined. Remove from heat and whisk in the sugar and both cocoas until combined and very smooth. Allow to cool briefly.

Meanwhile, whisk the sour cream, egg and vanilla together until smooth, then whisk into the warmed cocoa mixture. Whisk together very well.

Whisk together the flour, baking soda and salt in a large mixing bowl to aerate and combine. Add about a quarter of the liquid mixture and begin whisking to combine. Keep adding the liquid mixture to the dry, whisking well as you go. Make sure they batter is well mixed. It will be very fluid. You want to make sure there are no pockets of flour left behind. Scrape batter into prepared pan.

Bake for about 35 to 45 minutes or until a toothpick inserted in the center comes out with just a few crumbs clinging. The center might sink a bit; don’t worry! It will be covered with the delicious, creamy whiskey enhanced frosting.

For the Frosting: Cream the butter in a mixing bowl with an electric mixer until very smooth. Add the confectioners’ sugar in a few batches, beating well until smooth and creamy after each addition, adding the liquid as well, starting with using only 1 tablespoon of the milk. Beat until very smooth, creamy and spreadable. If it is a bit too thick, add a little lactose-free milk; if it is too thin add a little more confectioners’ sugar.