How to Make It
Preheat the oven to 350°. Line a 12-cup muffin tin with paper or foil liners.
In a medium bowl, whisk the flour with the cornstarch, baking powder and salt.
In a large bowl, using a handheld electric mixer, beat the sugar with the eggs and vanilla extract at medium-high speed until smooth and thickened slightly, about 3 minutes. Add the butter and oil and beat until incorporated, scraping the bottom and side of the bowl. Add the dry ingredients and milk in 3 alternating batches, beating well between additions. Carefully pour the batter into the lined muffin tins, filling them about two-thirds full.
Bake the cupcakes in the center of the oven for 20 to 23 minutes, until springy and a toothpick or cake tester inserted in the center comes out clean. Let the cupcakes cool slightly in the muffin tin, then transfer them to a wire rack to cool completely.
In a small saucepan, combine the granulated sugar with the water and bring to a boil over moderately high heat, stirring constantly until the sugar has dissolved, 2 to 3 minutes.
In a large bowl of a standing electric mixer, beat the egg whites with the pinches of cream of tartar and salt until soft peaks form.
With the mixer at medium speed, carefully pour the hot sugar syrup into the egg whites. Continue beating the frosting until cool and billowy, about 5 minutes. Mix the frosting with the lemon zest and juice to taste.
Preheat the broiler. Spread the frosting on the cooled cupcakes. Transfer the cupcakes to a baking sheet and broil for 30 seconds, or until the frosting is toasted; serve.