In a large pot, melt butter over low heat. Add 7 ounces of marshmallows and stir until melted and smooth.
Remove from heat and stir in vanilla extract. Quickly mix in 2 cups of crispy rice cereal until evenly coated.
Press the mixture evenly into the bottom third of the prepared pan. Allow to cool slightly.
For the strawberry layer, melt remaining 1.75 ounces of marshmallows with strawberry puree over low heat. Stir in 2 cups of cereal and press over the first layer.
For the chocolate layer, melt chocolate chips and remaining marshmallows together. Stir in remaining cereal and press into the final layer.
Let cool completely, then cut into squares and serve.
Notes
Store treats in an airtight container at room temperature for up to 3 days.