Place the white melting chocolates and the vegetable oil in a microwave safe bowl. Heat in 30 second intervals, stirring between each one until the chocolate if fully melted.
Stir the lemon zest into the melted chocolate.
Place the cereal in a large bowl and pour the chocolate mixture over the top. Gently stir to coat the cereal with the chocolate.
Then place the powdered sugar in a 1 gallon zip lock bag and pour the cereal in the bag over the top. Seal the bag tightly and shake to thoroughly coat the cereal with the powdered sugar.
Then pour the puppy chow on a baking sheet and allow to cool so that the chocolate can re-hardened.
Notes
Do not use lemon juice instead of lemon zest as this will cause the chocolate to harden and become clumpy.