In a large pot over low/medium heat, melt butter. Remove 1 cup of marshmallows from bag and set aside.Add remaining marshmallows to the pot of melted butter. Stir occasionally for 3-5 minutes until everything is completely melted.
Remove from heat and add vanilla and salt.
Add in fruity pebbles and carefully stir with a rubber spatula to combine everything.
Add the reserved cup of marshmallows. Transfer mixture to a greased 9x13 inch pan to set. Press on mixture until it's evenly distributed in the pan.
Place the white chocolate chips into a disposable piping bag or sandwich bag, and microwave for 15-second increments until melted, mixing between each. It usually takes 2-3 rounds.
Snip the tip of the bag and drizzle white chocolate over rice crispy treats. Optional: sprinkle with fun rainbow sprinkles! Let chocolate set for at least 10 minutes before slicing.
Notes
Store at room temperature for up to 5 days (best eaten within 3 days). Freeze for up to 3 months, separate with parchment paper to prevent sticking.