Creating the Sugar Cookie Lemonade Crumble involves several easy steps:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish.
2. Make the Sugar Cookie Base: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
3. Cream the Butter and Sugars: In a large mixing bowl, cream together the softened butter, granulated sugar, and powdered sugar until light and fluffy.
4. Add Egg and Vanilla: Beat in the egg and vanilla extract until well combined.
5. Combine Wet and Dry: Gradually mix the dry ingredients into the wet mixture until just combined. Do not overmix.
6. Spread the Dough: Evenly spread the cookie dough into the prepared baking dish. Use a spatula to smooth it out.
7. Bake the Cookie Layer: Bake in the preheated oven for 20-25 minutes, or until lightly golden. Allow it to cool completely in the pan.
8. Prepare the Lemonade Layer: In a mixing bowl, combine fresh lemon juice, granulated sugar, cold water, and lemon zest. Stir until the sugar dissolves.
9. Pour Lemonade Over the Cookie Layer: Once the cookie base is cooled, pour the lemonade mixture evenly over the top.
10. Prepare the Crumble Topping: In a separate bowl, mix rolled oats, flour, brown sugar, and cinnamon. Add melted butter and stir until crumbly.
11. Add Crumble Topping: Spread the crumble mixture over the lemonade layer, ensuring even coverage.
12. Final Bake: Return the dish to the oven and bake for an additional 15 minutes or until the topping is golden brown.
13. Cool Completely: Allow the crumble to cool for at least 30 minutes before serving to let the layers set.
14. Cut and Serve: Once cooled, cut into squares and serve.