Step 1

Arrange a rack in the middle of the oven and heat the oven to 350°F. Line the bottom and sides of an 8x8-inch baking pan with 2 sheets of parchment paper, positioning them perpendicular to each other and making sure they are long enough to hang over all four sides by at least 1 inch to form a sling.

Step 2

Cut 1 stick unsalted butter into 8 pieces and place in a large microwave-safe bowl. Microwave in 20-second bursts until mostly melted, 40 to 60 seconds total. (Alternatively, melt the butter on the stovetop, then pour into a large bowl.)

Step 3

Add 3/4 cup packed light or dark brown sugar, 3/4 teaspoon baking powder, and 1/2 teaspoon of the kosher salt, and stir to combine. Add 1 1/4 cups old-fashioned rolled or quick-cooking oats and 1 cup all-purpose flour, and stir with a flexible spatula until no dry spots remain. Reserve 1 cup of the mixture for topping.

Step 4

Transfer the remaining crumble mixture into the baking pan. Pat into an even layer. Bake until fragrant and lightly browned, 12 to 14 minutes. Meanwhile, prepare the lemon filling.

Step 5

Wipe out the crumble bowl (no need to wash). Finely grate the zest of 2 large or 3 medium lemons (about 2 tablespoons) into the bowl. Juice the zested lemons until you have 1/2 cup, then add to the bowl. Add 1 (14-ounce) can sweetened condensed milk and the remaining 1/4 teaspoon kosher salt, and whisk until smooth.

Step 6

Pour the filling over the hot crust. Sprinkle the reserved crumble mixture evenly over the filling. Return the baking pan to the oven and bake until the edges are lightly browned and set, 22 to 24 minutes. Place the pan on a wire rack and let cool completely, about 1 1/2 hours. If the filling is softer than desired, refrigerate for 30 minutes once completely cooled to firm up.

Step 7

Grasping the excess parchment paper, lift the slab out of the pan and place on a cutting board. Cut into 12 pieces.