Prepare medium bowl with plastic wrap liner for batter and set aside
Place chocolate chips in mixing bowl and melt for 3-5 minutes / 45˚C / speed 2 and use the spatula, as needed, to ensure chocolate melts evenly (7 minutes for double batch)
Add butter to mixing bowl and melt for 2 minutes / 45˚C / speed 2 (4 minutes for double batch)
Place brown sugar, eggs and vanilla into mixing bowl and combine for 10 seconds / speed 3 (15 seconds for double batch)
Add cocoa powder, flour, baking powder and salt and blend for 10 seconds / speed 3, or until combined (10 seconds speed 3 twice for double batch) and transfer cookie batter to prepared medium bowl lined with plastic wrap to refrigerate for at least 2 hours, or overnight, until firm (cookie dough will be sticky and “batter-like“ so be patient and remove every lovely morsel of deliciousness from the mixing bowl)
Pre-heat oven to 325˚F or 165˚C and line baking tray with parchment
Roll dough into 12 balls* about 2 tablespoons each and flatten gently, then bake 6 cookies at a time for 10 minutes only (no longer) or freeze (see note)
Prepare chocolate pieces** with halved marshmallows while dough is baking
Remove from oven and quickly place a 7 gram portion of baking chocolate into middle of each soft partially baked cookie and top each chocolate portion with cut side of halved marshmallow (cut side down)
Return to the oven for 5 more minutes or until the marshmallow has puffed (may not be golden, but sure pretty if it is)
Remove immediately and top with optional chocolate shavings, rest 5-10 minutes and serve while still warm and gooey