Heat the oven to 190°C, 170°C fan, gas 5. Line two baking sheets with baking paper.
In a large bowl, cream together the butter and sugars until pale and fluffy then beat in the egg yolk and vanilla.
Add the flour and oats and stir together until the mixture forms a soft dough.
Shape the dough into 16 or 24 balls, depending on the size of cookie you would like. Place them evenly on the baking sheets, and flatten with a fork.
Bake for 7-12 minutes depending on size. Leave for a few minutes on the baking sheets to firm up then lift onto a wire rack to cool.
Sandwich together with a generous spoonful of Hazelnut Chocolate Spread. Best eaten within 2 days.
The sandwiched crumblies will freeze, so make a batch and use as needed. If you have no time to make your own cookies, buy a pack of crisp oat biscuits and sandwich them together with our Hazelnut Chocolate Spread for an instant sweet treat.