Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
Make the crust: Combine flour and powdered sugar. Cut in cold butter until mixture resembles coarse crumbs. Press into the prepared pan.
Make the lemon filling: In a separate bowl, whisk together sugar, lemon zest, lemon juice, and eggs.
Pour lemon filling over the crust.
Make the streusel topping: Combine flour, brown sugar, cinnamon, and cold butter. Use your fingers to rub the butter into the dry ingredients until it resembles coarse crumbs.
Sprinkle streusel topping evenly over the lemon filling.
Bake for 30-35 minutes, or until golden brown and a toothpick inserted into the center comes out clean.
Let cool completely before cutting into squares.
Notes
Store leftover squares in an airtight container at room temperature for up to 3 days.