Dissolve the cornstarch in 50ml whole milk, leave to rest.
In a small saucepan combine the leftover milk, the sugar and a pinch of salt, bring to an almost boil, add the cornstarch milk and cook constantly stirring until thickened. Take off heat, add the cookie butter, stir until smooth and incorporated. Cool to room temperature, then transfer to the fridge overnight.
Freeze in your ice cream maker according to the manufacturer's instructions adding biscoff cookie crumbs at the last couple of minutes of churning. Store in an airtight container or spoon right away.