In a medium bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon of baking powder, and 1/2 teaspoon of salt. In a separate large bowl, cream 1 cup of unsalted butter with 1 cup of granulated sugar until light and fluffy, about 2-3 minutes. Add in 1 egg and 2 teaspoons of vanilla extract, mixing until well combined. Gradually incorporate the dry ingredients, creating a smooth dough. Divide the dough into two discs, wrap in plastic, and chill in the refrigerator for at least 1 hour.

Preheat your oven to 350°F (175°C) while the dough chills. Once ready, take one disc of dough out of the fridge and place it on a lightly floured surface. Roll the dough to about 1/4 inch thick, ensuring an even surface that will make the cutting step easy. Keep your rolling pin dusted with flour to prevent sticking. Use a sharp knife or a spatula to transfer your rolled dough onto a parchment-lined baking sheet.

Using cookie cutters, cut out various shapes from the rolled dough to create your Stained Glass Cookies. For the 'glass' effect, use a smaller cutter to make a hole in the center of each cookie shape. Arrange the cookies on the baking sheet, leaving space between each cookie. Take assorted hard candies and crush them lightly, filling each hole with the colorful candy pieces to ensure they will melt and create the stained glass effect.

Place the baking sheets in the preheated oven and bake the cookies for 8-10 minutes. Keep an eye on them—the edges should be lightly golden, and the candies will begin to melt, forming a vibrant glass center. Once baked, remove the cookies from the oven and allow them to cool on the baking sheet for a few minutes before transferring them to a wire rack.

Let the cookies cool completely on the wire rack. This step is crucial, as the melted candy will be hot and delicate right after baking. Once cooled, the candy centers will harden, creating that spectacular stained glass look.