Preheat oven to 180 degrees. Line and grease an 8 inch brownie pan and set aside.
Allow the melted butter to cool.
Whisk together the flour, cocoa powder, sugar, baking powder and salt till there are no lumps and is a smooth mixture.
Add the butter, milk and yogurt and fold in till no lumps. The batter would be glossy and would move out of sides of the bowl.
Spread the batter evenly on the prepared pan. Drop the Biscoff spread on different places on the top of the brownie batter and swirl to evenly distribute. Crush the cookies and press down onto the batter. (refer notes)
Bake for 30 minutes. Cool in pan for 15 minutes and then allow it to cool on a rack till completely. Slice and enjoy!
Notes
I feel it is better to fold in the crushed cookies for the ease of cutting. However, the cookies on the top do add to a lovely crunchy layer and texture.