Beat butter until smooth, then add sugar and beat 5 minutes until pale and fluffy.
Yolks Add egg yolks one at a time, mixing well after each.
Dry Whisk flour, baking powder, and salt. Mix into batter on low just until combined.
Flavor Stir in rum (or vanilla).
Prepare Grease tart pan and add parchment round.
Spread Press and smooth thick batter evenly into the pan.
Score Use a fork to lightly make a crosshatch pattern on top.
Wash Mix egg yolk + water and brush gently over the surface.
Bake Bake at 350°F (175°C) for 35–45 minutes until deep golden and set.
Cool Cool 15 minutes in pan, then remove and cool fully on a rack.
Dust Dust with powdered sugar and serve (berries optional).