- 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.
Use a stand mixer to get the fluffiest icing. (With a hand-held electric mixer, beat an extra 4 min.) Clean mixer bowl and whisk thoroughly before using — any trace of grease will prevent meringue from whipping properly.
Use the back of a cereal spoon to create smooth decorative peaks and swirls on the meringue.