Line or generously grease an 8x8" square baking pan.
In a saucepan, melt the butter on a low heat. Add the 10oz marshmallows and continue to melt over a low heat. Stir constantly.
Once the marshmallows have almost entirely melted, remove from the heat and stir in the fruity pebbles cereal and handful of mini marshmallows.
Press into the pan and refrigerate for at least 1 hour.
Once set, if using, melt the white chocolate and drizzle over the marshmallows treats. Slice and serve!