Add the brown sugar and granulated sugar and mix for another 1–2 minutes until well combined. Add the eggs and egg yolk, and mix again until the mixture is creamy and fully blended.
Add the flour and baking soda, and mix on low speed until the dough is almost combined.
Fold in the chocolate chips with a spatula until evenly distributed and the dough is just combined.
Cover the bowl with plastic wrap and chill the dough in the fridge for at least 1 hour. For best flavor and texture, chill overnight.
Use a cookie scoop (or about 1½ tablespoons of dough) to form evenly sized cookie dough balls.
Place the dough balls on the prepared baking sheet and bake for 11–12 minutes, or until the edges are lightly golden and the centers still look soft.
Immediately top the warm cookies with extra chocolate chips and Biscoff cookie chunks. You can also add small drops or a drizzle of melted Biscoff on top for extra flavor.
Let the cookies cool on the tray for 5 minutes before transferring them to a wire rack to cool completely.
Notes
Use room-temperature ingredients. Make sure the butter, Biscoff spread, and eggs are at room temperature so they mix smoothly and create a soft, even dough.
Chill the dough. Chilling for at least 1 hour helps prevent spreading and deepens the flavor. Overnight is even better.
Measure flour correctly. Too much flour makes cookies dry. Spoon and level your flour instead of scooping from the bag.
Don’t overmix the dough. Once you add the flour, mix just until combined to keep the cookies soft and chewy.
Pull the cookies out slightly underbaked. The centers should look soft; they finish setting as they cool.
Top while warm. Press extra chocolate chips or Biscoff cookie chunks on top right after baking for a pretty, bakery-style look.
Bake one tray at a time. This ensures even baking and keeps the cookies from spreading too much or baking unevenly.
Line your sheet pan with parchment paper or a silicone mat! This step helps the cookies to bake evenly, and it makes sure that your cookies release easily from the pan.
Use a cookie scoop. Scooping the dough with a cookie scoop helps the cookies bake evenly and stay the same size.
Add a Biscoff drizzle. For even more flavor, drizzle melted Biscoff on top of the warm cookies or add small Biscoff drops before baking.