Mix the flour, salt, and baking powder together.

In the mixer bowl, cream the butter and sugar for 2 minutes.
Add the egg and egg yolk and beat for another minute or two, then add the vanilla.
Reduce the mixer speed and add the flour mixture.
Work the dough just enough to make it pliable.

Wrap the dough in plastic wrap and refrigerate for 2 hours.

Center the rack in the oven and preheat to 350 F (180 C).
Prepare two cookie sheets by lining them with parchment paper or silicone mats.

On a floured surface, roll out the dough to a thickness of 1/4″ (0.5cm).
Cut out the cookies using a cookie cutter and place them on the baking sheets.

Bake the cookies for 9 to 11 minutes.

When they come out of the oven, let them rest for 5 minutes before transferring them to a cooling rack.

Royal Icing:
Using an electric mixer, mix the egg white and powdered sugar, and add food coloring.
If the mixture is too liquid, add more powdered sugar.
If the mixture is too thick, thin it out with a few drops of lemon juice.
Spread the mixture with a spatula or with a piping bag.