Preheat the oven to 350. Grease an 8x8 baking pan with cooking spray.
Melt the chocolate and butter together using a double boiler (or simply place a heat safe bowl over a saucepan filled with 1-2 inches of simmering water.) Set aside to cool.
4 oz dark chocolate, 14 tbsp butter
In a large bowl, whisk the eggs and then mix in the sugar and whisk until light and frothy.
3 eggs, 1 ¼ cups sugar
Add cooled melted chocolate/butter mixture to the bowl with the eggs and sugar. Whisk together.
In a separate small bowl, sift together the cocoa powder, flour, baking powder and salt.
½ cup cocoa powder, 6 tbsp flour, ¾ tsp baking powder, Pinch of salt
Add the dry ingredients to the wet mixture and stir until combined. Fold in the broken cookie pieces.
4 oz Biscoff cookie pieces
Pour the batter into the prepared 8x8 pan and bake for 30 minutes.
While still warm, add dollops of cookie butter to the top and spread out to frost evenly.