Heat your oven to 350°F.
Brown the butter in a small saucepan over medium-low heat until it starts to darken and the "popping" sound stops. Remove the pan from the heat.
1 cup (2 sticks or 227 grams or 8 ounces) unsalted butter
Butter a 9 inch square pan or line the pan with parchment paper and lightly butter the paper.
Sift or whisk the flour, cinnamon, salt, and baking powder, and baking soda into a medium sized bowl.
2 cups (250 grams or 8.8 ounces) bleached, all-purpose flour, 1 teaspoon salt, ½ teaspoon baking powder, ½ teaspoon baking soda, 1 teaspoon cinnamon
Add the sugars to a mixing bowl, add the brown butter without scraping any burnt bits at the bottom of the pan, and beat until combined. Let cool slightly if needed to avoid scrambling the eggs.
1 cup (192 grams or 6.6 ounces) dark brown sugar, ¼ cup (50 grams or 1.8 ounces) granulated sugar
Beat in the eggs and vanilla for one minute, one at a time, scraping down the side of the bowl as necessary.
2 large eggs, 1 teaspoon vanilla extract
Mix in the dry ingredients just until uniform, then transfer the batter to the pan.
Dollop the Biscoff on top of the batter. Then use a knife to spread and swirl the Biscoff into the batter, being careful not to go to the bottom of the pan.
½ cup Lotus Biscoff cookie butter
Bake for 30 to 35 minutes, until a toothpick inserted into the Biscoff blondies comes out clean and the blondies are just slightly firm to the touch.
If you used parchment paper, let the blondies cool in the pan for 10 minutes, then use the parchment to remove the Biscoff blondies to a rack.
Store at room temperature for 1 to 2 weeks. If they last that long.