Place the chocolate in a microwave safe bowl. Microwave in 30 second intervals, stirring in between, until melted and smooth. Set aside to cool.

In a medium bowl, sift together flour, cocoa, baking powder, baking soda and salt.

In a large bowl mix together oil, sugar, eggs. Beat until fluffy. Stir in vanilla and melted chocolate, and mix to combine.

Add the flour mixture all at once, and mix until just combined. Cover with plastic wrap and refrigerate until firm enough to roll into balls, at least one hour or overnight.

Preheat oven to 350 degrees. Line two large large baking sheets with parchment paper. Set aside.

Scoop heaping tablespoonfuls of dough and roll into balls.Place on prepared sheets spacing 2 inches apart. Top each ball of dough with about 6-8 marshmallows, pressing down the dough with the marshmallows to flatten. Bake 11-12 minutes until set, rotating the sheets halfway through. Let cookies cool on tray for 5 minutes, then transfer to wire racks to cool completely. Repeat with remaining dough.