In a bowl whisk together flour, baking powder, and salt; set aside.

Cream the butter and granulated sugar until light and fluffy, about 2–3 minutes.

Beat in the egg and vanilla until fully incorporated.

Gradually add the dry ingredients and mix until a dough forms; add 1–2 tbsp milk only if dough is too dry.

Flatten the dough into a disk, wrap in plastic, and chill in the refrigerator for 1 hour.

Preheat the oven to 350°F (175°C). Roll the chilled dough to 1/4 inch thickness on a lightly floured surface.

Cut shapes with cookie cutters and transfer to parchment-lined baking sheets.

Bake for 8–12 minutes until the edges are just set; remove and cool completely on a wire rack.

Make the royal icing: beat egg whites (or meringue powder + water) until frothy, then gradually add sifted powdered sugar and cream of tartar; beat to desired stiffness.

Divide icing into portions, color with gel food coloring and adjust consistencies: thicker for outlining, thinner (add a few drops of water) for flooding.

Pipe outlines on cooled cookies using thick icing; allow outlines to crust 10–15 minutes.

Flood the cookie centers with thinner icing, use a scribe or toothpick to smooth and remove air bubbles.

Let decorated cookies dry uncovered at room temperature for 6–24 hours until icing is fully set before stacking or packaging.