2 ¼ cups flour
½ teaspoon baking powder
¼ teaspoon salt
¾ cup butter, softened
¾ cup sugar
1 large egg
2 teaspoons vanilla extract
3 medium egg whites
4 cups powdered sugar
1 teaspoon glycerine
blue, green, red, and yellow food coloring
Whisk the flour, baking powder, and salt together in a medium bowl. Set aside.
Beat the butter and sugar together using a hand mixer until light and fluffy. Add the egg and vanilla, and beat until combined.
Fold the dry ingredients into the wet. Divide the dough into two equal parts, wrap in plastic wrap, and chill at least two hours and up to two days.
Once chilled, preheat the oven to 350 F. Roll the dough out to ¼-inch thickness. Cut into egg shapes using a cookie cutter.
Arrange the cookies on cookie sheets lined with parchment paper. Bake 11 to 12 minutes, until lightly browned around the edges.
Cool slightly, then transfer to a wire rack to cool completely.
Whisk the egg whites together, then add the powdered sugar by the tablespoonful, whisking all the time, until thick enough to fall off the spoon. Stop adding sugar, then using an electric beater, beat until the icing comes to stiff peaks. Stir in the glycerine.
Divide the icing into four bowls and color each one with a few drops of food coloring to create pastel colors. Using piping bags, decorate the cooled cookies with royal icing.
