Preheat the oven to 350°. Line 24 muffin cups with paper liners. Melt the butter in a small saucepan and keep warm over low heat.
In a small bowl, whisk the flour with the cornstarch. In a large stainless steel bowl, whisk the sugar, eggs, vanilla, orange zest and salt. Set the bowl over a pot of barely simmering water and whisk the mixture constantly until warm to the touch. Remove from the heat.
Using a handheld electric mixer, beat the mixture at high speed until pale, thick and tripled in volume, about 5 minutes. Sprinkle the dry ingredients on top and fold them into the batter.
Pour the warm butter down the side of the bowl, letting it sink to the bottom. Working quickly, fold the butter into the batter until blended. Immediately spoon the batter into the muffin cups until three-quarters full. Bake the cupcakes for about 12 minutes, until light golden and springy to the touch. Let cool slightly in the pans, then transfer the cupcakes to racks to cool.