Place the chopped dark chocolate in a heatproof bowl. In a small saucepan, gently heat the heavy cream until it just begins to simmer (small bubbles at edges), about 1–2 minutes; do not boil. Pour the hot cream over the chocolate and let sit 1 minute to soften. Stir gently until smooth and glossy. Stir in the 1 tablespoon butter, if using, until incorporated. Let the ganache cool at room temperature until thickened but still pipeable/spoonable, about 30–45 minutes. If it cools too much, warm briefly in 5–10 second bursts in the microwave. If using sea salt, stir a pinch into the ganache after it comes together.

Once thick (spreadable but not runny), transfer ganache to a piping bag or small container. For faster setting, chill briefly (10–15 minutes) but check often so it stays scoopable.

Cookie Dough

Prepare oven and pans

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 stand mixer fitted with the paddle attachment, beat the softened butter, granulated sugar, and brown sugar together on medium speed until pale and fluffy, about 2–3 minutes.

Add egg and vanilla

Add the egg and vanilla extract and beat until combined, scraping down the bowl as needed.

Combine dry ingredients

In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt until evenly combined.

Mix dry into wet

With the mixer on low speed, gradually add the dry ingredients to the butter mixture just until incorporated. Avoid overmixing.

Fold in chips

Fold in the semi-sweet chocolate chips by hand with a spatula so they are evenly distributed.

Portion dough and fill

Scoop about 2 tablespoon-sized portions of dough (approx. 24 portions from this batch). Flatten each portion in your palm into a disc about 2¾ inches across. Place about 1 heaping teaspoon of cooled ganache in the center of each disc. Bring dough edges up and around the ganache, sealing the filling inside and rolling gently to form a smooth ball. Ensure seams are well sealed so filling doesn't leak. Place seam-side down on the prepared baking sheet, spacing about 2 inches apart.

Optional chill

For thicker cookies and less spreading, chill the filled dough balls on the baking sheet for 15–20 minutes in the fridge.

Bake

Bake in the preheated oven for 10–13 minutes, until the edges are set but centers still look slightly soft. Rotate the pans halfway through baking for even color.

Finish

Remove cookies from oven and immediately sprinkle a tiny pinch of coarse sea salt on each cookie if desired. Let cookies cool on the baking sheet for 5–7 minutes, then transfer to a wire rack to cool completely. As they cool, the ganache will soften into a molten center again.