INSTRUCTIONS
[ FOR THE CAKE ]
1. Adjust oven rack to middle position and heat oven to 350 degrees. Grease 13 by 9-inch baking dish. In bowl of stand mixer fitted with dough hook, combine flour, milk, sugar, egg, yeast, and salt. Knead on low speed until dough comes together, about 2 minutes.
2. With mixer running, add butter 1 piece at a time, waiting until each piece is incorporated before adding next. Increase speed to medium-high and continue to knead until dough forms stretchy, web-like strands on sides of bowl, about 6 minutes longer (dough will be soft and sticky).
3. Using greased rubber spatula, transfer dough to prepared dish. Using your floured hands, press dough into even layer to edges of dish. Cover dish tightly with plastic wrap and let dough rise at room temperature until slightly puffy, about 1 hour.
[ FOR THE TOPPING ]
4. Ten minutes before dough has finished rising, whisk melted butter, granulated sugar, brown sugar, cinnamon, and salt together in bowl. Add flour and stir with rubber spatula or wooden spoon until mixture forms thick, cohesive dough; let sit for 10 minutes to allow flour to hydrate.
5. If dough has pulled away from sides of dish after rising, gently pat it back into place using your floured fingers. Break topping mixture into rough 1/2-inch pieces using your fingers and scatter in even layer over dough in dish. (Be sure to scatter all crumbs even though it may seem like too much.)
6. Bake until crumbs are golden brown, wooden skewer inserted in center of cake comes out clean, and cake portion registers about 215 degrees in center, about 35 minutes. Transfer dish to wire rack and let cake cool completely. Using spatula, transfer cake to cutting board; cut cake into 12 squares. Dust squares with confectioners' sugar and serve.
[ TO MAKE AHEAD]
7. Once dough has been pressed into even layer in baking dish and dish has been wrapped tightly in plastic wrap, dough can be refrigerated for at least 4 hours (to ensure proper rising) or up to 24 hours. When ready to bake, let dough sit on counter for 10 minutes before proceeding with step 4. Increase baking time to 40 minutes.