1 Preheat the oven to 175°C (350°F) and line two baking sheets with parchment paper.

2 In a bowl, whisk together the flour, cocoa powder, baking soda, and salt until evenly combined.

3 In a separate large bowl, cream the softened butter with both sugars until light and fluffy (about 2–3 minutes).

4 Beat in the egg and vanilla extract until incorporated.

5 Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Do not overmix.

6 Fold in the chocolate chips, mini marshmallows, and optional chopped nuts gently with a spatula.

7 Using a cookie scoop or tablespoon, drop rounded portions of dough (about 45g each) onto the prepared baking sheets, spacing them 5 cm apart. Press a few extra mini marshmallows or a chocolate chunk on top of each cookie for a gooey finish.

8 Bake for 10–12 minutes, until the edges are set but the centers still look slightly soft. The marshmallows will puff and turn lightly golden.

9 Remove from the oven and let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely. Sprinkle with flaky sea salt if desired.

10 Serve warm for maximum gooeyness. Store cooled cookies in an airtight container for up to 3 days.