Preheat your oven to 340°F and grease a cast iron skillet or oven-safe pan.
In a large bowl, whisk together the melted butter, Biscoff spread, and sugar until well combined.
Add the flaxseed and plant-based milk, stirring until smooth.
Mix in the flour, cocoa powder, protein powder, salt, and baking powder, using a spatula or wooden spoon until everything is fully combined. Fold in the chopped chocolate and Biscoff cookies.
Pour the batter into the prepared pan and spread it evenly.
Sprinkle a few more chopped cookies and chocolate over the top, pressing them lightly into the batter.
Bake for 30 minutes, or until the edges are set and the center remains gooey.
Remove from the oven and let cool for 5-10 minutes before serving.