Mix sugar, oats, and melted butter together in a bowl (use a spoon).
Beat the egg and add it to the dough. Mix everything well.
Stir flour, baking powder, vanilla sugar, and salt into the dough.
Use two teaspoons to place dollops of dough on a baking sheet with parchment paper. Remember to leave sufficient space between each cookie as they spread a lot.
Bake the cookies at 180 degrees Celsius (356 degrees Fahrenheit) (fan-forced) for about 5-6 minutes.
Allow the cookies to cool on a rack.
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Notes
Optionally, add a bit of finely grated lemon zest to the dough if you want a slightly different cookie flavor.