For the frosting: Whisk the sugar and flour together in a small saucepan. Mash 2 containers (about 2 1/2 cups) blackberries with the lemon juice in a small bowl with a fork. Strain through a fine-mesh strainer into the saucepan and whisk with the sugar-flour mixture until evenly combined. Bring the mixture to a boil, then reduce the heat to medium and cook, stirring constantly, until the jam is thickened to the consistency of thick pudding, about 3 minutes. Scrape the frosting base into a bowl and place a piece of plastic wrap directly on the surface of the mixture. Refrigerate until chilled, about 1 hour.
For the cake: Meanwhile, preheat the oven to 350 degrees F. Butter two 9-inch round cake pans, then line the bottoms with parchment paper and butter and flour the paper. Set aside.
Pour the corn and beer into a blender and puree until smooth, at least 1 minute. Reserve the puree in the blender.
Whisk the flour, baking powder, baking soda and salt in a bowl until evenly combined. Combine the butter, sugar and vanilla in the bowl of a stand mixer fitted with the paddle attachment and beat on medium speed until pale and fluffy, about 3 minutes. Add the eggs, one at a time, beating well after each addition, until smooth. Alternately, add the flour mixture and the beer-corn puree, beginning and ending with the flour mixture, until the batter is smooth. Divide the batter between the prepared pans, smoothing the tops with an offset spatula, then bake until a cake tester inserted in the center of the cakes comes out clean, 45 to 50 minutes.
Let the cakes cool in the pans for 20 minutes, then invert them onto a cooling rack and remove the parchment paper. Let the cakes cool completely, then split each in half horizontally so you have 4 even thin cake layers.
Beat the butter, vanilla and salt for the frosting with an electric mixer on medium speed until lightened and creamy, about 1 minute. Stir the frosting base with a rubber spatula, then add a third of it to the bowl and beat on medium-high speed until incorporated. Add the remaining base and beat until fluffy, lightened and whipped, about 3 minutes more.
Center a cake layer on a cake plate and spread about 1/2 cup frosting over the cake with an offset spatula or butter knife. Repeat with 2 more cake layers and frosting, then top with the last cake layer. Scrape the remaining frosting onto the cake and spread it evenly over the top and sides. Halve the remaining blackberries and decorate the top of the cake with them. Refrigerate the cake 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.)
Copyright 2016 Television Food Network, G.P. All rights reserved.