Mix the butter, sugar and salt in a mixing bowl.

Add the flour until the dough feels firm and comes nicely together.

Let the dough rest in plastic foil in the fridge for 1 hour.

Grease a 9-inch pie pan and set aside.

Preheat the oven to 400F.

Push the dough into the pie pan. You can make the dough even, using the back of a spoon or a butter knife. Make fun criss-cross lines with a fork.

Let the boterkoek rest for another 30 minutes

Beat the egg in a cup and brush over the boterkoek.

Put in the oven for about 20 minutes. Ovens can be different from each other, so check after 17 minutes. The boterkoek should be golden brown.