Base: Grease a swiss roll tray and line with baking paper.
Tip the rice bubbles into a medium sized bowl. In a heat proof bowl over a pot of boiling water, melt the marshmallows and butter. Stir until smooth, it takes a while to break down. Quickly pour the melted marshmallow into the rice bubbles and stir until combined.
Press the rice bubble mix into the prepared tray. If you wet your hands lightly, the mix won't stick to you.
Topping: Cover the rice bubble base evenly with the mini marshmallows and press in lightly.
In a heat proof bowl over a saucepan of boiling water, melt the chocolate, cream and butter. Stir until smooth. Drizzle over the marshmallows.
Refrigerate until set then cut into pieces. Store in fridge.