Preheat the oven to 325 degrees F.. Grease a 9x13-inch baking dish with butter or non-stick spray. Set aside.
In a large mixing bowl combine 1/2 cup confectioner's sugar and 2 cups all-purpose flour. Whisk until combined.
Add in the butter. Using a mixer, beat until it is well-combined. Stir in crushed almonds and almond extract. Mix well to evenly distribute the almonds.
Spread the dough evenly over the bottom of the prepared baking dish. Pat down firmly. Bake for 15 to 20 minutes. While the dough bakes, prepare the filling.
In a large mixing bowl, combine the sugar and 1/4 cup all-purpose flour.
Add in the eggs, baking powder and lemon zest. Mix well until thoroughly combined into a smooth, lump-free filing. Spoon the filling over the hot dough. Spread and smooth evenly.
Place back into the oven and bake for 25 to 30 minutes until golden brown. The center may slightly jiggle.
Allow to cool completely. Slice into bars, then cover them with a generous layer of confectioner's sugar if desired and more lemon zest if desired.