• Preheat oven to 350°F. Lightly grease or spray a 13x9-inch baking pan. Line the pan with parchment paper if desired to help with easy removal. Set aside.

To prepare the crust:

• Pour the sugar, flour, baking powder, salt and lemon zest in the bowl of a food processor. Pulse until combined. Add the butter and process until the mixture resembles coarse sand. Add the egg and process again until the dough starts to hold together but still crumbly. (NOTE: If you don’t have a food processor you can use a pastry blender.)
• Gently press two-thirds of the dough into the bottom and slightly up the sides of the prepared pan.

To prepare the lemon filling:

• In a large mixing bowl whisk together the sour cream and sweetened condensed milk until combined. Add the egg yolks, lemon juice and zest. Whisk until blended. Pour the filling into the prepared crust.
• Using your hands, gently press together handfuls of the remaining crumb topping to create clumps. Scatter the clumps and remaining crumbled topping over the lemon layer leaving some filling showing through.
• Bake for 40 to 45 minutes or until the edges are light brown.
• Cool completely before slicing. Once cooled, cover and refrigerate until ready to serve.