Preheat the oven to 350°F (180°C) and line two large baking sheets with parchment paper.
Place the butter and half of the chocolate (¾ cup or 120 grams) into a small saucepan (use a double boiler if needed) and melt over very low heat. Whisk* to combine and set aside to cool slightly.
Place the flour, cocoa, baking powder, baking soda, salt, and vanilla powder (if using, not extract) into a bowl and whisk to combine. Set aside.
Add the sugar and eggs to a large bowl and beat with a hand mixer for five minutes, or until very smooth and light in colour.
Beat the cooled melted chocolate into the sugar and egg mixture until combined.
Stir in the flour mixture with a wooden spoon or spatula until just mixed – don’t over mix. The dough will be very soft, almost like cake batter.
Gently fold the remaining chocolate and mini marshmallows into the dough.
Cover and let the dough rest for thirty minutes.
Scoop the dough out onto the prepared baking sheets, leaving about 5 cm (2 inches) between each cookie.
Bake for 10-12 minutes, or until the cookies are cracked and the edges are just firm to the touch.
Cool the cookies for about ten minutes on the pan before removing to cool fully on a wire rack. These will keep in an airtight container for about a week and freeze well.
Video
Notes
* If using vanilla extract, use 1 teaspoon and add it to the chocolate mixture after melting.If you have a fan-forced oven, both batches can be baked at once. Otherwise do one pan at a time.