Prep the pan. Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal. Lightly grease the parchment.

Melt the butter. Melt the butter in a large microwave-safe bowl or saucepan. Let it cool for 5–10 minutes so it is warm, but not hot.

Mix the wet ingredients. Add the Biscoff cookie butter, light brown sugar, and granulated sugar to the melted butter. Whisk until smooth and glossy. Add the eggs, egg yolk, and vanilla extract, then whisk until fully combined.

Add the dry ingredients. Add the flour, cinnamon, baking powder, and salt. Fold together with a rubber spatula until just combined. Do not overmix.

Fold in the cookies. Gently fold in the chopped Biscoff cookies, saving a small handful for topping if desired.

Assemble the blondies. Spread the batter evenly into the prepared pan. Warm the extra Biscoff cookie butter for 10–15 seconds, just until drizzleable. Dollop it over the top of the batter, then use a butter knife to gently swirl it in. Sprinkle with the reserved chopped cookies and cinnamon sugar.

Bake. Bake for 30–34 minutes, or until the edges are set, the top looks slightly crackly, and the center no longer looks wet. A toothpick inserted into the center should come out with a few moist crumbs, not wet batter.

Finish and cool. Sprinkle with flaky salt, if using. Let the blondies cool completely in the pan before lifting them out and slicing. For the cleanest cuts, chill them for 30–60 minutes before slicing.