1. Preheat oven to 375ºF. Mist 12 cups of a muffin tin with cooking spray.
2. In a large bowl, whisk together egg, sugar, lemon zest, juice, vanilla and salt until well blended.
3. Gently fold flour and baking powder into egg mixture with a rubber spatula until just incorporated. Mix in butter. Divide batter among cups, filling each 1 about halfway. Bake until cakes have risen and turned golden, about 11 minutes. Immediately run a knife around edges and invert cakes onto a wire rack to cool slightly. Serve warm, dusted with confectioners' sugar, if desired.