1. Preheat the oven to 350°F 180°C and grease a donut pan with cooking spray or butter.
2. Place the softened butter and brown sugar in a large bowl and beat for 2 minutes with a hand whisk or electric mixer until creamy and smooth.
3. Add the egg and vanilla extract and mix again to incorporate.
4. Add the cookie butter spread and milk and mix until smooth.
5. Finish with the dry ingredients, cake flour, biscoff cookie crumbs, baking powder and salt and mix again with a hand whisk or soft spatula to incorporate.
6. Pour the batter into the greased donut pan and bake for 8-10 minutes, until puffed and cooked in the center.
7. Leave to cool in the pan before carefully removing from the mould and cooling completely on a cake rack.

Icing

1. Heat biscoff butter spread in a medium bowl in the microwave.
2. Dip each donut in the spread until halfway covered with glaze.
3. Then place on a cooling rack to drain off excess glaze.
4. Heat white chocolate in the microwave or in a bowl over a double boiler, and place melted white chocolate in a piping bag.
5. Decorate the top of the glazed donuts by making stripes with the white chocolate, then immediately sprinkle with crushed biscoff cookies.
6. Leave to cool to room temperature and enjoy!