In a large sauce pan, melt butter and 1½ bags of marshmallows; reserve the other half of the bag for later.
When butter and marshmallows have melted together, add cocoa pebbles and stir.
Cool one minute and stir in the rest of the marshmallows.
Pour into a buttered 9x13 baking dish and press down. In a microwave safe bowl melt milk chocolate chips with 1 teaspoon shortening or oil. Microwave in 30 second increments, stirring between each increment until melted and smooth. Do the same thing with white chocolate chips and remaining 1 teaspoon shortening or oil.
Drizzle melted milk chocolate and white chocolate over the top of treats.
Let set for 1 hour before cutting into.
