Line a 9-by-13-inch baking dish with enough foil so that there's a 2-inch overhang on all sides. Grease the foil with butter.
Melt the butter in a large pot over medium heat. Add the marshmallows and honey and cook, stirring occasionally, until the marshmallows melt completely, about 5 minutes. Remove from the heat and stir in the vanilla and a pinch of salt. Add the cereal and 1 cup each of the crushed strawberries and bananas. Stir with a rubber spatula, making sure to scrape up the bottom, until the cereal is completely coated. Transfer to the prepared baking dish and pat down into an even layer. Scatter the remaining crushed fruit on top and press into the top (use the bottom of a measuring cup to help). Let sit at room temperature until firm, about 30 minutes.
Cut into 24 bars. Store at room temperature in an airtight container for up to 3 days.
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