Preheat oven to 375F. Mist 12 cups of a muffin tin with cooking spray.
In a large bowl, whisk together egg, sugar, lemon zest, juice, vanilla and salt until well blended.
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.