Cream softened butter and granulated sugar until light and fluffy, then beat in the egg and vanilla extract.

In a separate bowl, whisk together flour, baking powder, and salt, then gradually add the dry ingredients to the wet mixture until just combined.

Divide the dough into two discs, wrap in plastic, and chill in the refrigerator for at least one hour.

Preheat oven to 375°F (190°C), roll out the chilled dough on a floured surface to about 1/4 inch thickness, and cut into desired Christmas shapes.

Place cookies on baking sheets and bake for 8-10 minutes until edges are lightly golden, then transfer to a wire rack to cool completely.

In a large bowl, whisk together the powdered sugar and meringue powder.

Gradually add warm water and vanilla extract, beating on low speed until combined, then increase speed to high and beat for 5-7 minutes until stiff peaks form.

Adjust icing consistency by adding more water for a thinner flood consistency or more powdered sugar for a thicker outlining consistency.

Divide the prepared royal icing into separate bowls and tint with food coloring as desired.

Using a piping bag with a small round tip, outline each cooled cookie with a thicker icing, then immediately flood the inside with a thinner icing.

Allow the flood icing to dry completely (several hours or overnight) before adding any intricate details or additional layers of decoration.