I started by placing the very soft butter in a bowl and stirring it until smooth. Then I added the powdered sugar and vanilla, mixing by hand until fluffy. After that I incorporated the flour and cornstarch until the dough came together.
Next I turned out the dough onto a lightly floured surface, formed a ball and flattened it into a disk. Then I rolled it to roughly ⅜‑inch thickness (a helper tip: use adjustable rolling rings) and used cookie cutters for festive shapes. I transferred the shapes to sheet pans, then chilled them in the refrigerator for at least one hour so that they keep their shape when baking.
Preheat your oven (about 350 °F / 175 °C) and bake the chilled cut‑outs for about 12‑14 minutes until the edges just begin to turn golden. Then let them cool completely on a wire rack. While they were cooling, I prepared the glaze and buttercream piping. I dipped the cookies top‑side down into the glaze (letting excess drip), flipped them, then used a piping bag to draw little green tree squiggles and added sprinkles (nonpareils) for extra charm.