Step 1

Preheat oven to 375 degrees F (190 degrees C). Grease an 8-inch square baking pan or line with parchment paper.

Step 2

Melt butter, hazelnut liqueur, and chocolate-hazelnut spread together in a small saucepan over medium heat, stirring occasionally, until starting to bubble, about 5 minutes. Remove saucepan from heat and cool slightly.

Step 3

Whisk flour, cocoa powder, and salt together in a bowl.

Step 4

Beat white sugar, brown sugar, eggs, and vanilla extract together in a large bowl using an electric mixer until smooth and creamy; slowly mix in butter mixture until smooth. Stir flour mixture into butter-sugar mixture just until batter is combined; fold in chocolate chips, if using. Pour batter into the prepared pan.

Step 5

Bake in the preheated oven until the top is dry and the edges have started to pull away from the sides of the pan, 20 to 25 minutes. Cool brownies in pan for at least 2 hours before cutting.