For the Cake: Grease a 13-by-9-inch metal baking pan with cooking spray. In the bowl of a stand mixer fitted with the dough hook attachment, whisk together flour, sugar, yeast, salt, and lemon zest, if using. In a liquid measuring cup, whisk together warm milk and egg.

Start mixer on low speed and slowly pour in milk mixture in steady stream. Mix, scraping down sides and bottom of bowl as needed, until dough is uniform and no dry flour remains, 2 to 3 minutes.

Increase mixer speed to medium. With mixer running, add softened butter 1 piece at a time, mixing to incorporate after each addition. Continue to knead until dough is stretchy and webby, about 8 minutes longer (dough will be soft and sticky).

Using a greased dough scraper or rubber spatula, transfer dough to prepared baking pan. Lightly grease your hands and press and stretch dough into an even layer to fill the edges of pan. Cover pan tightly with a kitchen towel or greased plastic wrap and let dough rise at room temperature until slightly puffy, 45 minutes to 1 hour.

For the Crumb Topping: Meanwhile, adjust oven rack to middle position and preheat oven to 350°F (275°C). Clean and dry stand mixer bowl and fit stand mixer with paddle attachment. In mixer bowl, combine softened butter, brown sugar, granulated sugar, and cinnamon. Cream together on medium-high speed until light and fluffy, about 5 minutes. Stop mixer and using a rubber spatula, scrape down sides of bowl. Start mixer on medium-low speed and add flour, a few tablespoons at a time, until incorporated and mixture forms clumps with the texture of soft cookie dough. Scrape dough from paddle, cover bowl, and let sit at room temperature for at least 10 minutes.

When dough is ready, remove plastic wrap. If dough has shrunk away from sides of pan, gently stretch it back into edges. Use your hands to squeeze and compress crumb topping into large chunks, then break into ½- to 1-inch pieces and scatter evenly over dough in pan. (It will seem like way too much, but it’s not!) Using flat hands, gently press crumb layer into surface of cake.

Bake until crumb topping begins to turn light golden brown and wooden skewer inserted in center of cake comes out clean, 25 to 30 minutes. Transfer baking pan to wire rack and let cake cool completely, about 2 hours. Cut cake into squares and dust heavily with confectioners’ sugar right before serving.