Combine butter, egg, and sugar. Place the butter, egg yolks, and sugar into a large mixing bowl.
Cream together and add flavoring. Beat on medium speed with a hand mixer for 2-3 minutes until light. Add the vanilla extract and lemon zest and mix until combined.
Add dry ingredients. Add the flour, cornstarch, baking powder, and salt.
Make into dough. Mix just until combined and the dough forms.
Roll, chill, and cut the dough. Transfer the dough to a silicone mat or large cutting board and roll out to ¼-inch thickness with a rolling pin. Cover the rolled-out dough with plastic wrap and freeze for 20 minutes. Preheat the oven to 350 degrees F and line a cookie sheet with parchment paper. Use a star-shaped cookie cutter to cut shapes out of the sugar cookie dough and transfer these to the prepared baking sheet using a thin metal spatula.
Bake cookies and cool. Bake for 8–10 minutes in the preheated oven. Allow the cookies to fully cool on the baking sheet before decorating.
Melt chocolate. Melt the white chocolate in the microwave in 20-second intervals, stirring in between to avoid burning. You can use the "melt chocolate" setting on your microwave if it has one. Alternatively, place the chocolate in a heat-safe medium bowl over a pan of simmering water, keeping the bottom of the bowl from touching the water, and stir the chocolate until fully melted.
Coat cookies in chocolate. Dip the top of each cookie into the melted white chocolate.
Decorate cookies. Place the dipped cookies back onto the cookie sheet, then sprinkle with the sliced pistachios and decorate with metallic sugar pearls.
Chill cookies. Refrigerate for 10–15 minutes. The chill time is important to ensure the chocolate is set properly so that the decorations remain in place.