Prepare the Sugar Cookie Dough:

1. Cream Butter and Sugar: In a large mixing bowl, beat the softened butter and granulated sugar together until light and fluffy, about 3-5 minutes.
2. Add the Egg and Vanilla: Beat in the egg and vanilla extract until well combined.
3. Mix Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, mixing until a dough forms.
4. Chill the Dough: Divide the dough into two portions, wrap each in plastic wrap, and chill for at least 30 minutes in the refrigerator. Chilling helps prevent the cookies from spreading too much while baking.

Roll and Cut the Cookies:

1. Preheat the Oven: Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. Roll the Dough: On a lightly floured surface, roll out one portion of the dough to about 1/4-inch thickness. Use a heart-shaped cookie cutter to cut out the cookies and place them on the prepared baking sheet.
3. Bake the Cookies: Bake the cookies for 8-10 minutes, or until the edges are lightly golden. Let the cookies cool completely on a wire rack.

Decorate the Sugar Cookies:

1. Make the Icing: In a small bowl, whisk together powdered sugar and milk until smooth and thick enough to pipe. If you want colored icing, add food coloring to the icing mixture.
2. Pipe the Icing: Transfer the icing to a piping bag with a fine tip. Pipe the icing onto the cooled cookies, outlining the heart shape and filling it in with a smooth layer of icing. Feel free to get creative with different designs and colors.
3. Allow to Dry: Let the decorated cookies dry completely before storing or serving.