Weigh or measure the powdered sugar into a bowl and set it aside. Next, weigh or measure the flour, baking powder, and salt into another bowl. I run a whisk through the dry ingredients to blend them together.
Cream the cubed butter and cream cheese together on medium speed for 2 to 3 minutes until smooth and well blended.
Scrape down the sides of the bowl and add the powdered sugar. Mix on the lowest speed until incorporated, then increase the speed to medium and mix for 2 to 3 minutes until smooth and creamy.
Add the egg and vanilla extract, then mix until fully incorporated.
Gradually add the flour mixture, about one-quarter at a time, mixing on low speed just until combined. Do not overmix the dough. If needed, use a spatula to finish mixing any flour left on the bottom or sides of the bowl.
Cover the bowl and refrigerate the dough for 1 hour.
Preheat the oven to 350°F (175°C or 180°C). Line two cookie sheet pans with parchment paper.
Remove the cookie dough from the refrigerator. Using a medium cookie scoop, scoop the dough into 1 ½-inch balls. Roll each dough ball in superfine sugar until completely coated, then place them about 1 ½ inches apart on the prepared cookie sheet. Using two fingers, gently press the top of each dough ball to flatten it slightly.
Bake for 10-12 minutes. The cookies will be white with just a hint of golden brown around the edges. They may look slightly underbaked, but leaving them on the cookie sheet for about 1 minute helps them firm up before transferring them to a cooling rack to cool completely.
Once the cookies have cooled completely, add a little powdered sugar to a small sifter and lightly dust the tops of the cookies, if desired.