Add the butter to a small saucepan over medium heat. Cook, stirring occasionally, until milk solids turn brown and toasty, about 5 minutes.
Pour the butter into a large bowl and let cool to room temperature, about 30 minutes.
Meanwhile, preheat oven to 350 F (175 C). Spray an 8-inch square baking pan with nonstick spray and line the bottom with enough parchment paper that it goes up two sides.
To the cooled brown butter, add the brown sugar, Biscoff spread, and salt. Stir to combine. Whisk in the eggs and vanilla. Fold in the flour.
Dollop and spread the batter into the prepared pan. Press the cookies into the top (see Note 1).
Bake until set and edges are golden brown, 25 to 30 minutes. A few sticky crumbs are okay but not wet batter.
Transfer pan to a rack and cool completely. Cover with foil and let it firm up for at least 2 hours and up to overnight.
Using the parchment, lift the cooled blondie from the pan. Slice into 16 portions.
Microwave the remaining 2 tablespoons (39g) Biscoff until loose and pourable, about 15 to 30 seconds. Drizzle over the blondies.