Preheat oven to 350F and line an 8x8 pan with parchment paper. (If using a 9x9 pan see notes below on baking time).
In a mixing bowl, mix together melted butter, white and brown sugar until combined. Add the egg and vanilla extract and mix well. Whisk in the lemon juice.
Using a spoon or spatula, mix flour, baking powder, cinnamon and salt into wet ingredients, stirring until everything comes together and no clumps of flour remain (it will be quite thick like cookie dough).
Transfer the batter into the prepared pan and smooth out the top so the batter reaches the edges of the pan.
In a small bowl mix together 2 tbsp sugar with 2 tsp ground cinnamon. Sprinkle on top of the batter.
Bake for 22-27 minutes or until a toothpick inserted in the center comes out clean. Cool, cut into 9 squares and enjoy!
Notes
BAKING PAN: If using a 9x9 baking pan, blondies will not take as long to bake. Start checking doneness around 20-22 minutes of cooking time.