Line a 13 x 9-inch pan with aluminum foil and spray with cooking spray.
Combine corn syrup and sugar in a large microwave-safe bowl. Cook on HIGH (100%) for 2 to 2-1/2 minutes until sugar is dissolved and mixture is just starting to boil around the edges. OR cook in large saucepan over medium heat. Add 1 cup white chocolate chips and vanilla and stir until completely melted and mixture is smooth.
Pour over cereal in a large bowl and stir to coat completely. Pour into prepared pan; moisten hands with cold tap water and pat firmly in place.
Cool at least 30 minutes. Lift foil from pan and place on large cutting board. Cut in half horizontally to create 2 long rectangles. Diagonally cut off corners and then cut remaining into equal sized triangles (trees), alternating direction. Place trees on waxed paper.
Place remaining white chocolate chips and shortening in a small resealable plastic bag. Microwave on MEDIUM (70%) for 1-1/2 to 2 minutes until just melted. Cut off a small portion of one corner. Squirt a small amount of chocolate onto candy and press against tree as a stump. Drizzle remaining chocolate over the trees as "garland".