Preheat oven to 180 degrees top/bottom heat (convection oven: 160 degrees). Line a baking tray with baking paper and place a baking frame on it. Set the baking frame to 20 x 30 cm.
Crush the seeded oat cookies in a multi-shredder and mix with 2 tablespoons sugar in a small bowl. Melt the butter in a small saucepan. Mix the melted butter well with the cookie crumbs. Pour the cookie mixture onto the baking sheet and press down with a spoon.
Finely chop the dark chocolate coating, place in a metal bowl with butter and melt over a hot water bath. In a mixing bowl, beat eggs with sugar until foamy. Slowly add the oil and mix in a pinch of salt.
Add the melted dark chocolate-butter mixture to the eggs and beat until creamy. Mix the flour and baking cocoa in a separate bowl and sift into the batter. Then mix everything well.
Pour the chocolate brownie batter onto the cookie sheet and bake in the preheated oven for about 40 minutes.
After baking, let the brownies cool briefly, then spread mini marshmallows directly on the brownie. Either broil with the broil function in the oven for about 5 minutes or use a flambé to flambé the mini marshmallows on the brownie so that the marshmallows are lightly browned and melted. Cut brownie into about 24 pieces.