Combine first 3 ingredients in a heavy saucepan; whisk in eggs and milk, and cook, whisking constantly, over low heat until thickened (about 25 minutes). Remove pan from heat, and stir in butter and vanilla. Set custard aside.
Spread 1/2 teaspoon peanut butter between 2 chocolate wafers, forming a sandwich; repeat with remaining peanut butter and chocolate wafers.
Place sandwich cookies in the bottom of a 3-quart bowl; top with banana slices. Pour custard over bananas; cover and chill 4 hours.
Spread whipped topping evenly over custard, and sprinkle with peanuts before serving.