Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats—this keeps cookies soft on the bottom and easy to lift.

In a large bowl, beat the butter, granulated sugar, and brown sugar with a hand mixer on medium speed for 2–3 minutes, until light and fluffy. You’ll know it’s ready when the mixture leaves ribbon-like trails.

Crack in the egg, add vanilla, and mix until just combined—avoid overbeating or the cookies can puff up then deflate.

In a separate bowl, whisk together flour, cocoa powder, and salt. Slowly add the dry mix to the wet, stirring with a spatula until no streaks remain. The dough will feel soft but slightly firm.

Cover the bowl and chill in the fridge for 20–30 minutes. Chilled dough keeps its shape better and prevents the cookies from spreading too thin.

Using a 1-inch cookie scoop or tablespoon, portion dough into balls. Roll each in the extra granulated sugar, then set 2 inches apart on the prepared sheets.

Press your thumb (or the end of a wooden spoon handle) gently into the center of each dough ball to form a well about ¼-inch deep. Don’t press too hard—leave a bit of dough around the edges.

Bake for 10–12 minutes. The edges should look set but centers still a bit soft—cookies firm up as they cool. Let them rest 5 minutes on the pan.

While cookies bake, heat cream in a small saucepan over medium heat until simmering. Pour over chocolate chips in a heatproof bowl. Let sit 2 minutes, then stir until silky. Allow ganache to cool until slightly thickened but still pourable.

Use a tsp or small piping bag to fill each thumbprint with ganache. For extra flair, sprinkle a pinch of flaky sea salt or press a few mini chocolate chips on top. Let cookies cool completely so centers firm up, about 30 minutes.