Assemble equipment: stand mixer with paddle attachment (or large bowl and electric hand mixer), medium (2-inch) cookie scoop, baking sheet(s), Silpat or parchment (or cooking spray), wire rack, and mixing spoon or spatula.

In the mixer bowl, add 1 large egg, ½ cup softened unsalted butter (1 stick), ½ cup granulated sugar, ½ cup packed light brown sugar, and 2 teaspoons vanilla extract. Cream on medium–high speed until the mixture is light and fluffy, about 5 minutes. Stop once or twice to scrape down the sides and bottom of the bowl.

Reduce mixer to low speed. Add 1 ¾ cups cake flour (use cake flour as written), 2 teaspoons cornstarch, ¼ teaspoon baking powder, ¼ teaspoon baking soda, and a pinch of salt (optional). Mix only until the dry ingredients are just incorporated, about 1 minute. Do not overmix.

Add the entire 10-ounce bag of Reese’s Peanut Butter Chips and mix on low just until the chips are evenly distributed. The dough will be sticky and tacky.

Using a medium (2-inch) cookie scoop, form heaping two-tablespoon mounds of dough (about 13 mounds). Place the mounds on a large plate or tray, flatten each mound slightly with your fingers or the back of a spoon, cover tightly with plastic wrap, and refrigerate for at least 3 hours and up to 5 days. Do not bake warm dough (warm dough will spread and bake too thin).

When ready to bake, preheat the oven to 350°F (177°C). Line a baking sheet with a Silpat or parchment paper, or lightly spray with cooking spray.

Remove chilled mounds from the refrigerator and arrange them on the prepared baking sheet, spacing at least 2 inches apart (about 8 cookies per standard sheet).

Bake at 350°F for about 8–9 minutes, or until the cookie edges have set and the tops are just beginning to set (centers may still look slightly pale and glossy). Do not overbake; cookies will firm as they cool.

Remove the baking sheet from the oven and let the cookies cool on the sheet for about 5 minutes, then transfer the cookies to a wire rack to finish cooling.

While the cookies cool, make the frosting. In the mixer bowl, combine ⅓ cup creamy peanut butter and ¼ cup softened unsalted butter (half a stick). Beat on medium–high speed until light and fluffy, about 3 minutes.

With the mixer running on low, slowly add 1 ½ cups confectioner’s sugar and a pinch of salt (optional). Mix until the frosting comes together and is smooth.

If the frosting is too thick, add a splash of cream or milk, 1 teaspoon at a time, until you reach a spreadable consistency (author used about 1½ tablespoons total).

Add about 2 tablespoons of frosting to the top of each cooled cookie and smooth with a knife or offset spatula.

Optionally garnish each frosted cookie with chocolate sprinkles. Serve immediately.