Preheat your oven to 350°F (175°C). Grease or line an 8×8-inch baking pan with parchment paper.

In a medium bowl, whisk together flour, baking powder, and salt. In a separate large bowl, beat softened butter, granulated sugar, and brown sugar until light and creamy.

Add the egg and vanilla extract to the butter mixture, mixing well. Gradually add in the dry ingredients, stirring until just combined.

Press dough evenly into the bottom of your prepared pan.

In a medium bowl, whisk together lemon juice, granulated sugar, flour, and salt until smooth.

Add eggs one at a time, whisking well after each addition. Stir in melted butter.

Pour the lemonade mixture over the sugar cookie base.

In a small bowl, combine flour and granulated sugar with cold butter pieces.

Use a pastry cutter or your fingers to mix until the mixture resembles coarse crumbs. Sprinkle crumble evenly over the lemonade filling.

Bake for 30-35 minutes, or until the filling is set and the crumble topping is golden brown.

Allow to cool completely in the pan before cutting into squares.

For an extra touch of sweetness, sprinkle the top with additional granulated sugar before serving.