Place the butter, salt and sugar in a bowl. Whisk them together using a whisk or you can also use a stand mixer with the whisk attachment. Whisk until they look very creamy. If you're using a stand mixer, remember to pause every couple of minutes to scrape down the sides and bottom of the bowl to make sure everything is mixing correctly.
Add the egg and whisk again until it's creamy once more.
Add the mix and vanilla and mix.
Add the flour and baking powder and mix using a spatula if you're making it by hand or keep using the whisk attachment on your stand mixer.
Wrap the dough in cling film and refrigerate it for 30 minutes.
Remove the dough from the fridge. If it's too cold and hard, let it come a bit to room temperature beforehand.
First roll half of the dough using a rolling pin on a lightly floured surface and keep the other half in the fridge, unless it's rock hard. Roll it out until it's 2-3mm thick, adding more flour to the surface if needed.
Cut the cookies using a cookie cutter. First, cut out the edges of all the cookies. Then, count them and use a smaller cookie cutter to remove the centre of half the cookies. Gather the extra dough and roll it back out. If it has dried out too much, damp your hands to knead it together. Repeat the same with the other half of the dough until you have used up all of it.
Bake the cookies in a preheated oven at 180°C / 350°F for 10 minutes or until they are very lightly golden on the edges.