Preheat oven to 350 degrees and line an 8x8 or 9x9 square baking pan with non-stick foil (this way you can lift the blondies right out of the pan and cut them). Set aside.
Cream butter until light and fluffy, about 1 minute or so. Add in granulated and brown sugar and beat until light and fluffy.
Add in eggs, one at a time, beating until well after each addition.
Add in flour, baking powder and salt; mix until well combined.
Mix in vanilla extract, lemon zest and lemon juice.
Spread batter (batter will be thick) evenly in prepared baking pan and bake for 25-26 minutes, or until blondies are lightly golden brown and no longer wet in the center.
Allow to cool.
