Preheat oven to 180 C / Gas 4. Grease a 23cm square baking tin.
In a medium bowl, mix together the oil, sugar and vanilla. Beat in eggs. Combine flour, cocoa, baking powder and salt; gradually stir into the egg mixture until well blended. Spread the brownie mixture evenly into the prepared tin.
Bake for 10 minutes.
In a bowl, break digestives into small 1cm pieces and toss with chopped marshmallows and chocolate. Set aside.
After the brownies have baked for 10 minutes, remove from the oven and sprinkle the topping over the top. Bake for an additional 15 to 20 minutes, or until a skewer inserted in the centre comes out clean. Allow brownies to cool before cutting into squares.
Make individual brownies by spooning the brownie mixture into a mini muffin tin. Bake as directed, but reduce baking time as necessary.