prep: Preheat oven to 180C (160C fan)
Line a 20 cm square baking tin with parchment.
Melt base: In a heatproof bowl set over a bain marie, gently melt the butter and chocolate together. Stir until smooth, then remove from heat and let cool slightly.
Add sugar: Stir the caster sugar into the warm chocolate mixture until combined.
Add eggs: Beat the 2 whole eggs and 1 yolk in a small bowl, then whisk them into the chocolate-sugar mixture until glossy and smooth.
Fold in flour: Sift in the flour and gently fold with a spatula until just incorporated (do not overmix).
Add the rocky road mix-ins: Fold through most of the marshmallows, walnuts, Oreos, and dried fruit, reserving a handful for topping.
Bake: Pour batter into the prepared tin.
Scatter the reserved mix-ins over the top.
Bake for 25 - 30 minutes, until the edges are set but the centre still has a slight wobble (a skewer should come out with a few moist crumbs).
Cool & set: Allow to cool fully in the tin before cutting into squares.
The brownies will firm up as they set.