Weigh or measure the powdered sugar in a small bowl and set it aside. In another bowl, weigh or measure the flour. Add the baking powder and salt, then run a whisk through the mixture to blend.
In the mixer bowl, cube the butter. Cream on a medium mixer speed for 1 to 2 minutes, until soft, creamy peaks form. Add the powdered sugar and mix for 2 to 3 minutes. Next, add the vanilla extract and vanilla bean paste.
Scrape down the bowl and add the flour mixture about half at a time, mixing just until incorporated. Add the colored jimmies, then scrape down the sides of the bowl and transfer the dough to a pastry mat lightly sprinkled with powdered sugar.
Knead the dough just enough to incorporate any loose flour, then roll it into an 18-inch log. Cut the log in half and wrap each 9-inch log in plastic wrap. To help keep their rounded shape, set the logs in a baguette pan. Refrigerate for 1 hour.
Preheat the oven to 350°F (175°C or 180°C). Line two cookie sheet pans with parchment paper. You will also need a cutting board and a sharp knife for slicing the logs.I like a little crunch on the outside of my cookies, so I bring the cookie logs out of the refrigerator about 30 minutes before baking to let the outside soften slightly. I add sanding sugar or sparkling sugar to a dish wide enough to fit the 9-inch log. Because these cookies have a sandy, crispy texture, the dough is a little dry. I gently press the sugar into and around the log. I do not cover the entire outside, just enough to give the cookies a little crunch.
Slice the log into ⅛-inch cookies and place all the slices from one log onto one prepared pan. They do not spread, so you can place them fairly close together. I fit 23 slices on my first pan. Bake for 10–12 minutes, or just until the cookies are set to the touch. Move them to a cooling rack as soon as they come out of the oven.