Lemon Cupcakes with Vanilla Custard Filling

Steps

1. Make the filling first so it can chill: in a saucepan, whisk the filling portion of the granulated sugar (40 g), the cornstarch and the filling egg (1 egg) until smooth, then gradually whisk in the whole milk.
2. Cook the custard over medium heat, whisking constantly, until it bubbles and thickens and reaches at least 160°F (71°C), about 5 to 7 minutes.
3. Off the heat, whisk in the filling portion of the butter (10 g) and half of the vanilla. Scrape into a bowl, press plastic wrap onto the surface and chill at least 2 hours, until cold and set.
4. While the custard chills, heat the oven to 350°F (175°C) and line a 12-cup muffin pan with paper liners.
5. In a bowl, whisk together the cake flour, baking powder, baking soda and salt.
6. In a large bowl, rub the lemon zest into the batter portion of the granulated sugar (150 g) with your fingers until fragrant, then add the batter portion of the butter (115 g) and beat until pale and fluffy, about 3 minutes.
7. Beat in the batter eggs (2 eggs) one at a time, then the remaining half of the vanilla.
8. Stir the batter portion of the lemon juice (30 g) into the buttermilk. Add the flour mixture to the butter mixture in three parts, alternating with the lemon buttermilk in two parts, mixing on low just until smooth.
9. Divide the batter among the liners, about two-thirds full. Bake 18 to 20 minutes, until a toothpick in the center comes out clean.
10. Cool the cupcakes in the pan 5 minutes, then on a rack until completely cool, about 1 hour.
11. Cut a small cone from the top of each cupcake with a paring knife. Whisk the chilled custard smooth, spoon it into the hollows and replace the tops, trimming them flat.
12. Make the glaze: whisk the powdered sugar with the glaze portion of the lemon juice (25 g) until thick but pourable. Spoon over the cupcakes and let set 15 minutes. Keep leftovers refrigerated.