Swedish Flop

In the bowl of a stand mixer, combine yeast and milk; let stand 5 minutes. Whisk in butter, sugar, eggs, zest and vanilla. With the paddle attachment, beat in flour and salt until shiny, 3-4 minutes (batter will be very loose and sticky). Place in a greased bowl; lightly sprinkle surface with additional flour. Cover and let rise in a warm place until doubled, about 1 hour., Lightly grease a 13x9-in. baking pan; line pan with parchment, letting ends extend up sides. Transfer dough to prepared pan; cover and let rise until doubled, about 45 minutes. , Preheat oven to 350°. Meanwhile, for streusel, in a bowl, combine flour, brown sugar and salt. Add butter and rub together until coarse crumbs form; refrigerate while dough rises. , Sprinkle streusel evenly over dough. Bake on the center rack until edges start to brown, about 30 minutes. Cool in pan on a wire rack. , For frosting, whisk flour, milk, sugar, and salt in a large saucepan. Cook, whisking constantly, over medium heat until mixture thickens, about 1 minute. Remove and let cool completely. Whisk butter and extracts on high speed until fluffy, about 2 minutes. Add cooled flour mixture; beat until fluffy and mousselike, about 5 minutes. , Remove cake from pan. Using a serrated knife, cut 1/4 in. off outside edges; slice cake horizontally into 2 layers. Spread frosting evenly over bottom layer; top with jam. Replace top of cake; dust with confectioners' sugar.