Preheat oven to 180 °C (350 °F). Line two/three baking trays with parchment paper and set aside.
Sift together flour, cocoa, baking powder, and salt in a medium bowl.
Place chopped chocolate and butter into heat-proof bowl over a saucepan of gently simmering water. Allow to melt, stirring occasionally until fully melted. Remove from the heat and set aside to cool slightly.
Using an electric mixer (hand held or stand mixer), whisk together the eggs, brown sugar and vanilla on medium speed for 5 minutes, pour in the chocolate mixture and mix for 1 minute.
Add flour mixture and mix on low speed for 20 seconds until just combined.
Fold in mini marshmallows, chopped chocolate and crushed digestive biscuits (graham crackers).
Using a 2-3 Tbsp cookie scoop form the cookies, spacing cookies at least 2-3 inch apart (I baked 6 per tray).
The batter thickens quickly, so scoop all of the cookies immediately, and then bake the pans separately.-> Read my tips and tricks section in the post above!
Bake 10-12 minutes until tops are shiny and crackly. The cookies will be very soft so allow them to cool on the baking trays for at least 15 minutes before transfering them to a wire rack to cool completely.