1. Whisk sugar, cornstarch and salt together in a medium saucepan. Gradually whisk in half-and-half, then yolks. Place over medium heat and cook, whisking constantly, until pudding thickens and begins to boil, 3 to 5 minutes. Remove from heat, add butter and vanilla and whisk until butter has melted. Transfer to a 2-cup measuring cup that has a pouring spout.
2. Slice bananas and place a few on bottom of 6 4-oz. ramekins or custard cups. Divide 1/3 of pudding among cups, spreading over banana slices. Top each cup with 2 coarsely broken Nilla wafers. Repeat layers twice, ending with pudding. Cover each dish, pressing plastic directly onto pudding. Chill for at least 2 hours.
3. Just before serving, top with additional banana slices and sprinkle with Nilla wafer crumbs, if desired.