These traditional Dutch biscuits are known as zandkoekjes, literally "sand cookies", named for their delicate, crumbly texture. If you already make Breton sablés, you'll find them very familiar. Dutch zandkoekjes are usually a little plainer and less rich than some Breton version (recipe also on this site), allowing the buttery flavour to shine through.
Cream the butter and sugar together until smooth and pale.
Beat in the egg yolk, vanilla extract (if using) and salt.
Add the flour and mix until a soft dough forms. Do not overwork.
Shape into a flat disc, wrap and chill for at least 30 minutes.
Preheat the oven to 170°C fan (190°C conventional).
Roll the dough to about 8mm thick on a lightly floured surface.
Cut into rounds or traditional rectangular shapes and place on lined baking trays.
Brush lightly with the egg glaze. If desired, score a simple criss-cross pattern with a fork.
Bake for 12–15 minutes until lightly golden around the edges.
Allow to cool on the tray for 5 minutes before transferring to a wire rack.
Notes
Many Dutch bakers add a few flaked almonds to the top before baking, creating amandel zandkoekjes. For a more traditional Dutch bakery appearance, press a whole blanched almond into the centre of each biscuit before glazing.