Begin by lightly spraying 2 round pans (9 inches) with nonstick cooking spray and set them aside.
In a large pot, melt your butter. Keep stirring until the butter just starts to brown. Watch for the butter to foam up twice and emit a nutty aroma. Remove it from heat immediately to avoid burning.
Stir in 2 teaspoons of vanilla extract. The mixture will bubble up, so keep stirring for about thirty seconds until the sizzling subsides.
Add in half of your marshmallows, mix until almost melted, then add the remaining marshmallows. Stir until all the marshmallows are entirely melted.
Add green dye until you achieve the desired color. Then, add Rice Krispies, 2 cups at a time, and lightly stir until combined. If it starts sticking to the spatula, spray it with nonstick spray. Continue adding Rice Krispies until everything is evenly combined.
Pour the mixture into the prepared pans. Lightly wet your hands and gently press into the pan edges. Be gentle so you don't crush it!
Break up the white chocolate and place it in a heatproof bowl. Microwave for 30 seconds, stir, and repeat until completely melted. Let it sit for 3 minutes to cool to room temperature.
Using a spoon or piping bag, drizzle white chocolate onto the treats. Sprinkle on rainbow nonpareils, and you're ready to enjoy your festive Rice Krispie Christmas Trees!