For the frosting: Whisk the sugar, flour and salt together in a medium saucepan, then stir in the milk. Place the pan over medium heat and bring to a boil, stirring constantly. Continue boiling, stirring constantly, for 3 minutes (this cooks out the raw flour taste). Remove the pan from the heat and scrape the base into a shallow bowl. Press a sheet of plastic wrap directly against the surface of the frosting base and refrigerate until chilled and thick, at least 2 hours.
For the cake: Meanwhile, preheat the oven to 350 degrees F. Butter a 13-by-9-inch metal pan. Line the bottom and sides with parchment paper, then butter and dust the paper with cocoa powder.
Whisk the flour, brown sugar, cocoa, malted milk powder, baking soda, baking powder and salt in a bowl until evenly combined. In another bowl, whisk the butter, eggs, vanilla and 2 cups of the stout until smooth. Pour the wet ingredients over the dry ingredients and whisk until the batter just comes together. Pour the batter into the prepared pan and bake until a cake tester inserted in the center of the cake comes out clean, 40 to 45 minutes.
Meanwhile, pour the remaining 3/4 cup stout into a small saucepan and bring to a boil over high heat. Cook until reduced to 1/3 cup, 5 to 6 minutes. Remove the pan from the heat and stir the granulated sugar into the beer until dissolved. Let the stout syrup cool.
Let the cake cool in the pan for 30 minutes, then invert it onto a cooling rack and remove the parchment paper. Flip the cake onto another cooling rack so that it is right-side up again. Poke several holes all over the cake with a toothpick or wooden skewer, then drizzle the stout syrup evenly over the cake. Let the cake cool completely, then slide it onto a serving platter or board.
Remove the frosting base from the refrigerator and let stand for 20 minutes to come to room temperature. Beat the butter for the frosting with an electric mixer on medium speed until lightened and creamy, about 3 minutes. Add a third of the base and beat until evenly incorporated. Add half of the remaining base, beat until smooth, then add the remaining base and the vanilla and beat until the frosting is fluffy and light, about 3 minutes.
Scrape the frosting onto the cake and spread it evenly over the top and sides with an offset spatula or butter knife. Sprinkle the candy bars over the top of the cake and refrigerate until the frosting has firmed, at least 30 minutes or up to overnight, before serving.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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