Heat the oven to 350F and butter an 8 x 8 inch baking pan.
In a large bowl, whisk together the melted butter and brown sugar until smooth.
Add the egg, egg yolk, vanilla, Biscoff cookie butter, and salt and whisk to combine.
Stir in the flour until just combined.
Fold in white chocolate chips and 1 cup of the chopped Biscoff cookies. Pour into the prepared pan and spread the thick batter evenly. Sprinkle the top with the remaining ½ cup of chopped cookies.
Bake 20 to 25 minutes, or until a toothpick inserted in the center comes out with moist crumbs. Please don’t overbake!
Let the blondies cool to room temperature in the pan and cut into bars.
*Lotus Biscoff is a brand of speculoos or speculaas cookies. Any cookie by any of those names will work in this recipe.*Biscoff cookie butter is sometimes sold as just "cookie butter" under other brand names, such as Trader Joe's.