Spray a 9×13” pan with cooking spray. Set aside.
In a large microwave-safe bowl, melt butter in 15-second increments until melted.
Add 3/4 of the marshmallows to the melted butter and continue melting in 30-second increments, stirring between each, until smooth.
Fold in Rice Krispies cereal and the melted marshmallow mixture until well combined.
Stir in the remaining marshmallows (these will not melt completely).
Press the mixture evenly into the prepared pan and allow to cool.
In a separate bowl, beat together the softened cream cheese, sweetened condensed milk, lemon juice, and vanilla until smooth.
If desired, add a drop of yellow food dye to the cheesecake mixture and mix thoroughly.
Pour the cheesecake mixture over the cooled Rice Krispie base and smooth the top with a spatula.
Refrigerate for 3-4 hours, or until the cheesecake layer is set.
Once set, cut into squares. Melt white chocolate and drizzle over the bars.
Garnish with lemon zest if using.