Line a 9x9-inch baking pan with parchment paper. Set aside.
Melt the butter over medium heat in a large light-bottomed Dutch oven or large pot.
1/2 cup unsalted butter, save butter wrapper
Add in 6 cups of the mini marshmallows (save the other 2 cups for later!) and stir until melted.
8 cups mini marshmallows, divided
Remove from heat and turn off the burner. Add in the vanilla and salt, stir.
1 tsp. vanilla extract, pinch kosher salt
Add in both cereals, stir until combined.
4 cups Rice Krispie Cereal, 2 cups Fruity Pebble Cereal
Stir in the remaining 2 cups marshmallows and stir just until mixed, leaving pockets of marshmallow in the mixture.
Transfer to the prepared pan. Gently press down using the reserved butter wrapper or spray with baking spray and gently press down in the pan into a smooth, even layer.
Allow to cool for 30-60 minutes prior to cutting. Cover if letting them sit out on the counter for more than 1 hour. Lift the bars out of the pan and cut into 16 bars.
Melt the white chocolate and oil in the microwave until melted, about 1 min.
1/2 cup Ghirardelli white chocolate melting wafers or white chocolate chips + ½ tsp canola oil
Drizzle over bars. Allow to set up. Garnish with crushed fruity pebbles for a finishing look & enjoy!