Bring the unsalted butter to room temperature so it's soft but still holds shape (about 30–60 minutes). Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats. Measure and sift the all-purpose flour, cornstarch, and salt together into a bowl.

Cream butter and sugars

In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter on medium speed for 30–45 seconds until smooth. Add the granulated sugar and powdered sugar and continue beating for 2–3 minutes until the mixture is pale and fluffy, scraping down the bowl as needed.

Add yolk and vanilla

Add the egg yolk and vanilla extract to the butter-sugar mixture. Beat on low to combine, then increase speed to medium and mix 20–30 seconds until incorporated and smooth. The yolk helps bind and enrich the dough without adding extra moisture from a whole egg.

Combine dry and wet ingredients

With the mixer on low speed, add the sifted flour mixture in two additions, mixing just until the dough comes together. Do not overmix. The dough should be soft but hold its shape; if it's very sticky, chill 10–15 minutes.

Shape cookies

For drop cookies: Use a small cookie scoop (about 1 tablespoon) or tablespoon to portion dough onto the prepared baking sheets, spacing them 1.5–2 inches apart. For piped or pressed cookies: Transfer dough to a piping bag fitted with a large star tip or use a cookie press and pipe onto sheets. Alternatively, roll into 1-inch balls and slightly flatten with the bottom of a glass. If you want a bright sugar edge, sprinkle a little granulated sugar over each cookie before baking.

Bake

Bake in the preheated oven for 10–12 minutes, rotating the pans halfway through. Cookies should be set at the edges and just starting to color very lightly on the bottoms. They will remain pale; do not overbake or they will lose the tender butter texture.

Cool

Remove from oven and let cookies rest on the baking sheet for 3–5 minutes, then transfer to a wire rack to cool completely. While warm they are delicate; they firm up as they cool.

Finishing (optional)

If dipping in chocolate: melt chocolate in a heatproof bowl over a small pot of simmering water (double boiler) or in short bursts in the microwave, stirring until smooth. Dip half of each cooled cookie into the melted chocolate, allow excess to drip off, and place on parchment. Let chocolate set at room temperature or speed up by chilling briefly in the fridge (10–15 minutes).

If serving plain: store cookies in an airtight container at room temperature for up to 5 days. For maximum crisp-edge, layer with parchment between layers. For softer cookies, add a slice of bread to the container to retain moisture.