Frosting

Preheat oven to 350F. Lightly oil or spray a 9-in. round cake pan and line bottom with parchment. Sift flour with cocoa, baking powder, soda and salt into a medium bowl. Beat butter with brown sugar in a large bowl, with an electric mixer on medium-high, 3 min. Add eggs, 1 at a time, beating well and scraping down sides of bowl as needed. Beat in 1 tsp vanilla. Using a wooden spoon, stir in one-third of flour mixture, then add half the stout. Repeat additions, ending with flour mixture. Stir just until evenly mixed. Pour batter into pan and smooth top. Bake in centre of oven until a cake tester inserted in centre of cake comes out clean, 40 to 45 min. Cool in pan on a rack for 10 min, then turn out onto rack. Remove parchment. Cool completely, about 30 min. Combine granulated sugar and water in a small saucepan and bring to a boil over medium-high. Cook, stirring occasionally, until sugar dissolves and mixture is clear. Stir in 1 tsp vanilla. Whisk egg whites with cream of tartar on high in the bowl of a stand mixer, until soft peaks form when beaters are lifted, 1 min. Slowly pour hot sugar syrup down the side of the bowl, beating until stiff peaks form, 2 to 3 more min. Assemble by setting cake on a cake stand. Pile frosting on top of cake, creating peaks and swirls.