Preheat oven to 425 degrees. Prep a cookie sheet with parchment paper.
In the bowl of a stand mixer fitted with a paddle attachment, whip up the butter until it's light and fluffy. Add in the brown sugar and granulated sugar, whip up again until it's creamed together, about 2 minutes.
Add in the egg, vanilla, lemon extract, lemon zest, and poppy seeds. Whip up to combine and then scrape down the sides of the bowl.
Add in the flour, salt, baking soda, cornstarch, and mix to combine (don't overmix). Toss in the blueberries, then gently stir to combine on low speed.
Split the dough in half, then in half again, then in half again. You'll have 8 large spheres. Bake 4 at a time (or if you're making smaller cookies, 8 at a time, etc) for 7-9 minutes. The center will be gooey but will continue to bake through a little during the cooling process. Let cool completely on a wire rack, then add on the glaze.