Place chopped dark chocolate in a heatproof bowl. Heat the heavy cream until steaming but not boiling (about 30–45 seconds in microwave or small saucepan). Pour hot cream over chocolate and let sit 1 minute. Stir gently until smooth and glossy. Stir in the optional 1 tsp butter until incorporated. Transfer ganache to a shallow container and refrigerate until firm enough to scoop and shape, about 45–60 minutes.

Once the ganache is firm, scoop or portion into 1-tablespoon (roughly 12–15 g) balls. Place on a parchment-lined tray and freeze or refrigerate until solid, about 15–30 minutes (freeze if you need faster firming). You should get approximately 24 filling centers.

Cookie Dough

Prep

Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone liners.

Cream butter and sugars

In a large bowl or bowl of a stand mixer fitted with the paddle, cream the softened butter, granulated sugar, and light brown sugar on medium speed until light and fluffy, about 2–3 minutes.

Add eggs and vanilla

Add the eggs one at a time, mixing briefly until incorporated. Mix in the vanilla extract.

Combine dry ingredients

In a separate bowl whisk together the all-purpose flour, baking soda, fine sea salt, and cornstarch until evenly combined.

Mix dough

With the mixer on low, gradually add the dry ingredients to the butter mixture until a soft dough forms and no streaks of flour remain. Scrape the bowl as needed.

Fold in chips

Using a spatula, fold in 1.5 cups semisweet chocolate chips evenly through the dough.

Portion and enclose filling

Using a cookie scoop (about 1.5 tablespoons) or by hand, portion about 40–45 g (approx. 1.5 tbsp) of dough. Flatten each portion into a 2–3 inch round in your palm. Place one chilled ganache ball in the center, then fold dough around it, sealing edges and rolling gently to form a smooth ball. Make sure the ganache is fully enclosed with no gaps. Repeat until all filling centers are enclosed. If dough becomes too warm, chill balls 10–15 minutes before baking.

Bake & Finish

Top and bake

Place cookie balls on the prepared baking sheets, spaced about 2 inches apart. Lightly press 2–3 extra chocolate chips onto the top of each cookie if desired. Bake in preheated 350°F (175°C) oven for 10–13 minutes, until edges are set and the centers still look slightly soft. Rotate baking sheets halfway through for even baking.

Cool and finish

Remove cookies from oven and sprinkle a light pinch of coarse sea salt on top of each while still warm. Let cookies cool on the baking sheet for 5 minutes so they set, then transfer to a wire rack to cool completely. The ganache will be molten when hot and will firm slightly as cookies cool, leaving a chocolate-filled center.

Store cooled cookies in an airtight container at room temperature for up to 3 days or refrigerate for up to 7 days. For best texture, bring chilled cookies to room temperature before serving, or rewarm briefly (10–12 seconds) in the microwave to soften the center.