Begin by slightly melting the Biscoff cookie spread in the microwave for about 15-20 seconds, making it easier to work with. Spread it onto a piece of parchment paper in a thin layer and place it in the freezer for at least 2 hours to firm up, ensuring a beautifully gooey center for your fudgy Biscoff brownies.

While the cookie butter is chilling, preheat your oven to 325°F (160°C). Grease an 8x8 inch baking pan with butter or line it with parchment paper.

In a medium saucepan, combine the chopped semi-sweet chocolate and unsalted butter over low heat. Stir continuously for about 5 minutes until melted and smooth. Remove from heat and let it cool slightly.

In a large mixing bowl, whisk together the large eggs, egg yolk, and white granulated sugar for about 5 minutes until light and fluffy. Add pure vanilla extract and canola oil, mixing until fully combined.

Carefully fold the cooled chocolate mixture into the egg mixture using a spatula until just combined. Gradually add all-purpose flour, cocoa powder, cornstarch, and salt, stirring gently until no dry ingredients remain. Fold in Biscoff cookie pieces and mini chocolate chips.

Pour half of the brownie batter into your prepared baking pan, smoothing it out evenly. Carefully place the frozen Biscoff cookie butter layer on top, then spoon the remaining batter over it.

Bake the fudgy Biscoff brownies in the preheated oven for 40-50 minutes, or until the edges are set but the center is slightly underbaked. Let them cool completely in the pan on a wire rack before slicing.