Preheat your oven to 350°F (175°C). Grease an 8x8 inch baking pan and line it with parchment paper, leaving some overhang for easy removal later.

In a large mixing bowl, combine the melted butter, granulated sugar, and brown sugar. Whisk until well combined and smooth. This step is crucial as it helps to dissolve the sugars, creating a fudgy texture.

Add the eggs one at a time, whisking well after each addition. This helps to incorporate air into the batter, which contributes to the brownies' rise.

Stir in the vanilla extract and mix until fully incorporated.

In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt. Sifting helps to aerate the dry ingredients and remove any lumps.

Gradually add the dry ingredients to the wet mixture, folding gently with a spatula until just combined. Be careful not to overmix, as this can lead to tough brownies.

Fold in the Biscoff spread until evenly distributed. If using, add the chocolate chips at this stage.

Pour the brownie batter into the prepared baking pan and smooth the top with a spatula.

Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking to maintain a fudgy texture.

Once baked, remove from the oven and allow to cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.

In a mixing bowl, beat the softened butter and Biscoff spread together until creamy and well combined.

Gradually add the sifted powdered sugar, mixing on low speed until incorporated. This prevents a sugar cloud from forming.

Add the milk and vanilla extract, then beat on medium speed until the frosting is smooth and fluffy. Adjust the milk as needed to achieve your desired consistency.

Once the brownies are completely cool, spread the Biscoff frosting evenly over the top using a spatula.

Drizzle additional Biscoff spread over the frosting for an extra touch of flavor and presentation.

Cut into squares and serve. Enjoy your delicious Biscoff brownies!