Preheat the oven to 170C. It seems low but is correct. The boterkoek remains soft, with higher temperatures it'll get very hard.

Grease the (spring)form.

Beat the egg with a fork in a separate bowl.

Mix the flour, sugar and salt in a bowl.

Put about half of the egg to the flour / sugar mixture, keep the rest of the egg.

Add the soft butter to the rest of the ingredients in the bowl.

Mash the butter and egg with a fork into the flour / sugar mixture. Tip: If the butter is too hard, cut it small with 2 knives.

Knead - when everything is covered in flour - the contents of the bowl with your hands to a smooth, uniform dough.

Press the dough flat into the greased tin, make sure it is equally thick.

Brush the dough with the remaining egg.

Create - with the blunt side of a knife or a fork - a diamond pattern on top of the boterkoek.

Put the boterkoek in the middle of the pre-heated oven.

Bake the boterkoek 35-40 minutes until golden brown (and cooked).

Cool the boterkoek outside the oven.

Cut the boterkoek when it is completely cool.