Preheat oven to 350° F and line a 9- by 13-inch pan with parchment, leaving flaps hanging over the side (to use as handles later).
Mix your no-longer-hot melted butter and brown sugar vigorously with a whisk, spoon, or stand mixer until evenly caramel-colored and smooth.
Add eggs one at a time, mixing them in thoroughly. Stir in vanilla, lemon zest, lemon juice, and mascarpone.
In a separate bowl, whisk together the flours, salt, and poppy seeds. Now mix them into the wet ingredients. Overmixing isn't too much of an issue, but there's no reason to go overboard.
Spread your thick batter evenly into your prepared pan, tapping it against the counter to get out any air bubbles. Bake for 35 to 40 minutes (depending on your desired gooeyness factor).
While the blondies bake, whisk together all the glaze ingredients until smooth.
Remove from oven and cool completely in pan—it's even a good idea to give them a rest in the fridge or freezing before slicing!