• Preheat your oven to 350°F and line baking sheets with parchment paper.

Make the Dough:

• In a large mixing bowl, cream together the softened butter and sugar until light and fluffy.
• Add the egg and vanilla extract, continuing to mix until well combined.
• Sift the flour, baking powder and salt into the butter and sugar mixture.
• Mix until a cohesive dough forms.

Form the Cookies & Bake:

• On a lightly floured surface, roll out the cookie dough to about 1/4-inch thickness. Use a Christmas tree-shaped cookie cutter to cut out the tree shapes.
• Using a smaller Christmas tree shaped cookie cutter, punch out the middle of each cookie to create a "window" for the stained-glass effect.
• Carefully transfer the tree-shaped cookies onto the prepared pans, leaving space between each cookie.
• Place the hard candies in separate plastic zip-top bags for each color and use a mallet or rolling pin to crush into small pieces.
• Fill the center of each cookie with the crushed candies, using one color per cookie. Make sure to fill the openings completely and evenly.
• Bake the cookies for 8-10 minutes or until the edges are lightly golden brown and the candy has melted and filled the centers.
• Remove from oven and let the cookies cool on the pan for about 5 minutes.
• Carefully transfer the cookies to a wire rack to finish cooling completely. Allow the melted candy centers to harden before serving or storing.