Put the peanut butter and chocolate into a heatproof bowl and set over a bowl of barely simmering water, being sure the bowl does not touch the water. Heat the peanut butter and chocolate until just melted; stir until very smooth. Remove from the heat and cool until room temperature and the mixture has thickened.
Transfer the chocolate mixture to a quart-sized resealable plastic bag or disposable pastry bag without a tip. Snip 1 corner with scissors to create a piping bag. Squeeze about 1/2 teaspoon of the chocolate mixture onto a banana chip and top with a second. Continue making banana chip sandwiches until the chocolate is used.
Let the chocolate set completely at room temperature. Mini sandwiches can be stored covered at room temperature or refrigerated. They won't last long, though!
Cook's Note: If the chocolate mixture gets too firm to pipe, warm it briefly over the simmering water in the bowl to loosen. If you have already transferred it to a piping bag, simply wrap your hands around the bag and hold it for a few minutes. The residual heat from your hands will soften the chocolate until it is easy to pipe.