Preheat your oven to 325°F (163°C) and line an 8x8 inch baking pan with parchment paper.

Spread the melted Biscoff cookie spread evenly onto the prepared pan and freeze for at least 2 hours.

Melt the unsalted butter and chopped semi-sweet chocolate in a bowl in the microwave, stirring until smooth.

In a mixing bowl, whisk together the sugar, eggs, and egg yolk until pale and fluffy.

Combine the cooled chocolate mixture, vanilla extract, and oil with the egg mixture, mixing on low until just combined.

Sift in the flour, cocoa powder, cornstarch, and salt, folding gently, then add cookie pieces and chocolate chips.

Pour half of the brownie batter into the pan, add the solidified cookie butter, then top with the remaining batter.

Bake for 40 to 50 minutes, checking for a toothpick to come out with moist crumbs.

Cool the brownies completely in the pan before lifting out and slicing into squares.