Preheat the oven to 425 degrees F. Line 1 large or 2 small jelly roll pans with aluminum foil, then spray with nonstick spray and arrange the saltines salt-side down in a single layer.
Melt the butter and brown sugar together in a medium saucepan and boil until it turns a caramel color, a few minutes. Remove from the heat and pour over the crackers, covering them evenly. Put the peanut butter in a microwave-safe bowl and microwave just until slightly loosened, about 20 seconds. Drizzle evenly over the caramel and spread with a small offset spatula. Bake, watching carefully, until just bubbly, 3 to 5 minutes.
Put the melting wafers in a microwave-safe bowl and melt according to package directions. Spoon dollops over the peanut butter caramel and smooth evenly with an offset spatula. Transfer the pan to the fridge or freezer until completely cold, then break up into pieces. Store in an airtight container at room temperature.