Coat a 9x13-inch pan with nonstick spray and place it nearby before starting the marshmallow mixture. A prepared pan lets you transfer the sticky mixture quickly while it is still easy to spread.

Warm the butter in a large pot over low heat, then stir in the marshmallows until they melt into a smooth coating; take the pot off the heat and mix in the pudding powder and powdered bananas before folding in the cereal and crushed vanilla wafers. The marshmallow mixture should look glossy and smell warmly of banana and vanilla, with every piece of cereal evenly covered.

Transfer the mixture to the prepared pan and level it with a buttered spatula, waxed paper, or lightly greased hands. Pat gently rather than pressing hard so the bars keep their airy pockets and chewy texture.

Let the treats cool, then drizzle with the melted chocolate and arrange the whole vanilla wafers on top before slicing into squares once the coating has set. The chocolate should feel firm to the touch so it does not smear across the clean-cut squares.