Combine the butterscotch pudding mix, condensed milk and 1 1/2 cups water in a bowl and use an electric mixer to mix well. Refrigerate until set up according to the package directions.
Whip the cream in another bowl until soft peaks form. Gently fold half of the whipped cream into the set pudding. Fold in the second half until it's light and fluffy, taking care not to overmix. Refrigerate until ready to assemble the banana pudding.
Preheat an outdoor grill or a grill pan to medium-high heat.
Brush the cut side of each banana with the melted butter. Grill, cut-side down, until grill marks appear, about 2 minutes. Flip and let cook an additional 30 seconds to 1 minute. Remove from the grill and let cool until cool enough to touch. Peel, then slice 1/4-inch-thick.
Line the bottom of a 9-by-13-inch baking dish with a layer of vanilla wafers. Layer on half of the butterscotch pudding, then half of the bananas, then a light sprinkle of crushed toffee bar. Repeat the layers, for a total of 2 layers. Refrigerate 4 hours or up to overnight before serving.