Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large bowl, cream together butter, granulated sugar, and brown sugar until light and fluffy.
Add eggs one at a time, beating well after each addition. Mix in vanilla, lemon zest, and lemon juice.
In a separate bowl, whisk together flour, baking powder, baking soda, and salt.
Gradually add dry ingredients to the wet mixture, alternating with sour cream. Mix until just combined.
Pour batter into prepared pan and spread evenly.
Make the streusel: Combine 1/2 cup sugar, 1/3 cup flour, 1/4 cup butter (cold), and 1 teaspoon cinnamon in a bowl. Mix until crumbly.
Sprinkle streusel evenly over the batter.
Bake for 35–40 minutes or until a toothpick inserted in the center comes out clean.
Cool in the pan before serving. Enjoy!