Preheat oven to 375F. Grease the bottom of a 9x9 inch baking dish with cooking spray. Set aside.
In a large bowl, add granulated sugar and lemon zest. Rub together with your hands until lemon is worked into the sugar and has started to release the oils and is fragrant. It will take a few minutes.
Cream together lemon sugar mixture and butter until light and fluffy, about 3 minutes.
Add egg and lemon juice to the mixture and beat for 2 minutes, until egg is fully incorporated.
Mix in baking soda, baking powder, poppy seeds, and salt and stir to combine.
Add the gluten-free flour blend in two parts, mixing in between, until flour is fully incorporated. Spoon dough into the prepared baking dish and spread with a spatula. If the dough is too sticky, spray a spatula with cooking spray to help spread it out.
Bake at 375F for 15 minutes, until the edges are lightly brown. Let cool completely before adding the glaze.
To make the glaze, combine powdered sugar, lemon zest and lemon juice in a bowl and whisk to combine, until no clumps of powdered sugar remain. Pour the glaze over the cooled cookie bars and spread with a spatula. Sprinkle poppyseeds over the top. Allow to set completely before cutting.