Preheat the oven to 325 degrees. Prepare baking sheets with parchment paper.
In a large bowl, beat the butter & sugar and light and fluffy using a hand mixer.
Add in the egg, egg yolk & vanilla. Beat until well combined.
In a separate medium bowl, whisk together the flour, baking powder & salt.
Add half of the flour mixture to the butter mixture and gently mix on low speed. Once the flour is about absorbed, add the remainder of the flour mixture and mix until just combined. Do not over mix and be sure to thoroughly scrape the sides and bottom of your bowl every so often while mixing.
To roll out the dough, start with about ¼ of the dough and roll out on a lightly floured surface (ensuring your rolling pin is also lightly floured).
Roll to a ¼ inch thickness. Cut using your desired cookie cutters. Use an offset spatula to lift the cookies off the surface and place the cookies on the prepared tray. If you are decorating with sprinkles, this is when you would sprinkle your cookies.
Bake for 11-13 minutes. Since all ovens vary, I suggest starting your timer with 10 minutes and see how they look. These cookies are done when they are barely golden brown on the edges.
Allow to cool on the pan for 5 minutes and transfer to a baking rack to finish cooling. Decorate as desired.
