In a large bowl, cream the butter and sugar with an electric mixer. Beat in the eggs and vanilla.
In a separate bowl, mix the dry ingredients - flour, baking powder and salt. Stir the dry mixture into the butter mixture, one cup at a time. Chill the dough in the fridge for 3-4 hours.
Preheat your oven to 350 deg F and line a baking tray with parchment paper.
Roll the chilled dough out and cut shapes or circles with a knife, or by using the rim of a glass.
If you wish to decorate your cookies, brush the tops with milk and sprinkle on some coloured sugar. If you wish to frost with royal icing, bake plain.
Bake for about 10-15 minutes or until the cookies are just beginning to brown around the edges.
Transfer to a wire cooling rack and cool. If you are using royal icing, cool completely before frosting.
Using a large bowl, mix 3/4 cup water with confectioner’s sugar and meringue powder. Use an electric mixer to mix slowly until peaks begin to form. The icing should have a thick consistency and be a pure white color. If your icing is fluffy or bubbly, it has been overbeaten and need to be left to settle, before using a spatula to beat and smooth out the frosting.
If you are colouring your icing, add up to 1 tbsp food coloring and mix with a spatula until the color is mixed throughout. Icing can be stored at room temperature, covered with plastic wrap.