Preheat the oven to 350F. Line the bottom of a 9x13 inch rectangular baking pan with parchment paper.
In a medium bowl, use a pastry blender to combine the flour, cold unsalted butter cut into small pieces, and icing sugar, mixing them into a crumbly mixture. (You can also make the crust in a food processor: add the flour, sugar, and butter cubes and pulse for 20 seconds.)
Press the mixture firmly and evenly into the bottom of the prepared baking pan.
Bake for 20 minutes until the crust begins to set. Allow the crust to cool for about 10 minutes.
In a large bowl, beat together the eggs, granulated sugar, and lemon juice.
Add the flour, baking powder, and lemon zest and beat until smooth.
Pour over the baked crust and return the baking pan to the oven for another 25 minutes until the topping is set and just beginning to brown around the edges.
Let cool completely, then sprinkle with icing sugar and cut into squares. The recipe makes 24 squares or 35 tiny ones.
Notes
A 9x13-inch glass pan is the best pan for these lemon bars.
Use real lemon juice for the best taste.
Freezing instructions: Lemon bars can be kept in the freezer for up to 3 months. Allow the bars to cool, and cut them into squares, but do not dust them with powdered sugar. Freeze them individually in plastic or aluminum foil and place them in a large freezer bag. Defrost them in the refrigerator and serve them dusted with powdered sugar.