Melt together the butter and sugar in a medium pot.
Remove from heat and add the remaining ingredients in order.
Strain into the prepared tart shell and bake at 250F until the custard completely sets, about 20 minutes.
Allow to cool completely before decorating or storing.
Setting this custard all the way is essential for stability in the hot summer months. Whip up some fresh meringue to finish this tart in the classic American fashion, or sprinkle with granular sugar and brûlée for added crunch.