Preheat the oven to 350F/175C and grease a square 9x9 inch baking tin. Line with parchment paper.
Melt the butter and chocolate in a bowl over a saucepan full of boiling water, ensuring the bowl doesn't touch the water. Stir until melted and smooth, then set aside to cool slightly.
In a small bowl, sift the flour, baking powder, cocoa powder and salt.
In a large bowl, whisk the eggs, oil, sugars and vanilla. Add the slightly cooled chocolate mixture and whisk until smooth.
Add the flour mixture, bit by bit, stirring after every addition to combine until all of the dry ingredients have been used. Throw in the broken up digestive biscuits, mix again and then pour the batter into the prepared pan. Squish the giant marshmallows on to the top leaving a little space in between each one.
Bake for ~40 minutes, until the sides of the brownies have set, the top is beginning to crackle and look glossy. If you want to check if they're ready, insert a skewer or toothpick into the centre. The batter to the skewer should not be wet but should have a good amount of crumbs clinging to it.
Transfer the pan to a wire rack without removing the brownies and let them cool completely.
Once cool, gently lift or slide the brownies out of the pan and cut them into squares.