Place a heat proof bowl over 1 inch simmering water or use a double boiler. Pour chocolate chips into the bowl, add shortening. Melt, stirring occasionally until completely smooth.( Alternatively, melt chocolate and shortening together in a microwave safe bowl, using the melt function on your microwave. Stir until smooth.)
Meanwhile, line a large baking sheet with waxed paper or a silicone baking mat. Spread 4 1/2 cups cereal and marshmallows on pan. Crush remaining 1/2 cup cereal and set aside.
Drizzle melted chocolate over cereal and marshmallows. Spread using an offset spatula until evenly distributed. Sprinkle with crushed cereal.
Chill for 1-2 hours. Break apart and enjoy.
Store in an airtight container at room temperature for up to 1 week.