In the bowl of an electric mixer, or with a hand mixer, cream butter and sugar together until smooth, about 1 minute. Add vanilla and salt then beat to combine. Spoon in the flour 1/4 cup at a time, until completely combined. Shape dough into a disc, then wrap in plastic wrap and chill in the refrigerator for at least 30 minutes, or overnight.
Preheat oven to 325 degrees and generously spray a mini muffin tin with nonstick spray. Divide the dough into 20, equal-sized balls (I divided the dough in half, then each half in half, then each quarter into five balls,) then place one ball into each muffin cup. Use your thumb to press the dough down and create a shell, pressing the dough up the sides a bit. Bake for 10 minutes, cool for 5-7 minutes in the tin, then pop the shells out with a spoon and cool completely on a wire rack. If the dough has not left an indentation in the center of the shell, press down lightly with the bottom of a shot glass as soon as the shells come out of the oven.
Meanwhile, transfer pudding and whipped cream into individual piping or a ziplock bags, and snip the ends off. Pipe 1-1/2 - 2 teaspoons pudding into the bottom of the shell, pipe a dollop of whipped cream on the side, then place a banana slice on top. Serve immediately. Store extras in the fridge for up to a day.