Remove butter from the refrigerator at least 30 minutes before starting so it becomes soft but still holds shape. Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats.

Cream butter and sugars

In a large bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter, granulated sugar, and light brown sugar together on medium speed until light, fluffy and pale in color, about 2–3 minutes. Scrape down the bowl as needed.

Add eggs and vanilla

Add the eggs one at a time, mixing on low speed until incorporated after each addition. Mix in the vanilla extract until combined. Do not overmix.

Combine dry ingredients

In a separate medium bowl, whisk together the all-purpose flour, baking soda, fine sea salt, and cornstarch (and cocoa powder here if making a chocolate base). Ensure the cocoa is fully lump-free if used.

Mix dry into wet

With the mixer on low speed, gradually add the dry ingredients to the butter mixture in two additions, mixing until just combined and no streaks of flour remain. Do not overwork the dough.

Mix-ins & Customizations

Choose and fold in mix-ins

Decide which mix-ins you want (chocolate chips, nuts, oats, dried fruit, M&M's, sprinkles). Measure chosen mix-ins according to the ingredients list. Fold them into the dough by hand with a rubber spatula until evenly distributed — typically about 3/4 to 1 cup total of mix-ins per batch. If using delicate candy or sprinkles, reserve some to press on top before baking so they remain colorful.

Optional: chocolate base variation

If you included cocoa powder in the dry mix, the dough will be chocolate-flavored. Taste a pinch of raw dough only if you used heat-treated flour or you accept the risk of raw egg; otherwise proceed to shaping.

Shaping & Baking

Portion the dough

Use a 1.5 tablespoon cookie scoop or a heaping tablespoon to portion dough into balls about 1 to 1 1/4 inches in diameter. Place dough balls 2 inches apart on the prepared baking sheets to allow spreading.

Optional: egg wash and finishing salt

If you want a shiny surface, lightly brush tops of dough balls with the egg wash. Press reserved candies or sprinkles gently onto the tops, if using. Sprinkle a small pinch of flaky sea salt on each cookie just before baking, if desired.

Bake

Bake in the preheated 350°F (175°C) oven for 9–12 minutes, rotating the pans halfway through. Cookies are done when edges are set and lightly golden but centers still look slightly soft — they will continue to firm while cooling.

Cool

Remove baking sheet from oven and let cookies cool on the sheet for 5 minutes to set, then transfer to a wire rack to cool completely. For softer cookies, remove earlier; for crisper cookies, bake for the longer time and cool on the rack.

Storage & Serving

Store cooled cookies in an airtight container at room temperature for up to 4 days. To keep cookies soft, place a slice of bread in the container (replace bread when it becomes stale). For longer storage, freeze cookie dough balls on a tray until firm, then transfer to a freezer bag for up to 3 months. Bake frozen dough at 350°F (175°C) adding 2–3 minutes to bake time.