In a bowl, mix flour and baking powder. In a medium pan over medium heat, warm milk with butter until butter is melted, stirring occasionally. In another bowl with a mixer on high speed, whip eggs, sugar and vanilla until foamy. Add flour and milk mixtures. Stir to mix, then beat until well blended.
Scrape batter into buttered pan.
Invert cake onto a rack and turn rounded side up. Cool 20 minutes. Cool 10 minutes. Pour through a fine strainer into a bowl, discard residue. Add rum, liqueur or orange juice.