Preheat oven to 350F (175C) and line a 9×9 baking dish with parchment paper

Prepare crust by stirring together flour, sugars, salt, and baking powder until completely combined.

Drizzle melted butter over the top and use a fork to toss together until mixture is completely combined and all of the dry ingredients are moistened. It may still be a bit crumbly, but if pressed between your thumb and index finger it should cling together easily.

Firmly and evenly press ⅔ of the crumb mixture into the bottom of the prepared pan. I like to use the back of a spoon to smooth the surface and ensure the dough is pressed together. Reserve remaining crumble mixture for topping.

Transfer to 350F (175C) oven and bake for 15 minutes, edges will be a light golden brown. While crust is baking, prepare lemon filling

Prepare filling by combining condensed milk, egg yolks, heavy cream, and salt in a large glass mixing bowl and whisking vigorously until thoroughly combined.

While whisking, drizzle in lemon juice until ingredients are smooth and thoroughly incorporated.

Once crust has finished baking, pour the filling evenly over the top of the hot crust. Sprinkle the lemon filling evenly with remaining crumble topping.

Return to 350F (175C) oven and bake until the top is beginning to turn a light golden brown and the filling is set, about 15 minutes (the center may be slightly jiggly in the center but the surface should be set and it should not be entirely liquidy).

Allow to cool completely then transfer to refrigerator and chill for at least 4 hours (preferably overnight) before slicing and serving.