Heat the oven to 350°F (175°C). Grease and line a 9-inch square baking pan with parchment, leaving enough paper over the sides to help lift out the cooled cake.

Make the crumb topping first by whisking the flour, granulated sugar, brown sugar, cinnamon, and salt. Stir in the melted butter just until large, irregular crumbs form, then set the topping aside.

Whisk the cake flour, baking powder, and salt together in a separate bowl. In another bowl, beat the softened butter and granulated sugar for 2–3 minutes, until light and creamy.

Beat the egg and vanilla into the butter mixture. Add the dry ingredients in two or three additions, alternating with the milk, and mix only until the batter is combined.

Spread the batter evenly in the prepared pan. Scatter the crumb mixture generously over the surface, pressing a few portions together if you prefer larger pieces.

Bake for 32–38 minutes, until the cake is golden and a toothpick inserted in the center comes out clean or with a few moist crumbs. Cool the cake in the pan before cutting, then dust with confectioners’ sugar if desired.