In a medium saucepan, melt butter over medium heat.
Add the diced apples and cook for a few minutes until the apples are softened.
Place the cooled apple mixture over half of each square, leaving at least a 1/2" border. Brush the egg wash on the edges of the pastry using a pastry brush. Bring the edges together and press to seal. Crimp tightly along the edges with a fork to seal – the fork tines will seal the apple turnovers and also add a decorative element. The apple turnovers can be frozen at this stage – see freezer instructions in notes below.
In a small bowl, mix the Lotus Biscoff spread with milk till a liquid glaze is formed. In case the glaze is still thick, add a few more drops of milk. Drizzle the glaze over the apple turnovers, along with some crushed Lotus Biscoff biscuits (if using), and serve hot on their own, or with custard and ice-cream.
Nutrition for the recipe is for 1 apple turnover (ice-cream or custard not included), which is also the serving size for one person.