Prepare the baking sheets: Line 2 large baking sheets with parchment paper. Set them aside.

Cream the butter and sugars: In a large bowl, beat the softened butter, granulated sugar, and brown sugar on medium speed for about 2–3 minutes, until pale, creamy, and fluffy.

Add the egg and vanilla: Beat in the egg until fully incorporated, then mix in the vanilla. Scrape down the sides of the bowl as needed.

Combine the dry ingredients: In a separate bowl, whisk together the flour, cornstarch, baking powder, baking soda, and kosher salt.

Finish the dough: Add the dry mixture to the butter mixture in two additions, mixing on low speed just until combined. Fold in the 1/2 cup (80 g) rainbow jimmies with a rubber spatula. The dough should be soft but thick enough to scoop without running.

Chill the dough: Cover the bowl and refrigerate for 30 minutes. This firms the butter and makes the cookies easier to shape.

Preheat the oven: Heat the oven to 350°F (175°C). Line the baking sheets again if needed.

Portion the cookies: Scoop the chilled dough into 18 portions, about 2 tablespoons each. Roll each portion gently into a ball and place them on the prepared baking sheets, leaving about 2 inches (5 cm) between cookies.

Shape the dough: Gently flatten each dough ball to about 3/4 inch (2 cm) thick. Keep the cookies relatively thick so the centers remain soft.

Bake until lightly golden: Bake for 10–12 minutes, rotating the sheet halfway through if your oven browns unevenly. The edges should look set and just barely golden, while the centers should still look soft.

Cool the cookies: Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack. Cool completely before glazing.

Make the glaze: Whisk together the powdered sugar, milk, and vanilla until smooth and thick but spreadable. If it is too thick, add milk 1 teaspoon at a time. If too thin, whisk in a little more powdered sugar.

Glaze and decorate: Spread a thin layer of glaze over each cooled cookie. Immediately sprinkle with the optional 2 tablespoons (20 g) rainbow jimmies before the glaze sets.

Let the glaze set: Allow the cookies to stand at room temperature for about 15–20 minutes, or until the glaze is no longer wet to the touch.