Preheat oven to 325ºF (160ºC). Line standard muffin tins with paper liners. Whisk together flour, baking powder, and salt.
With an electric mixer on medium-high speed, cream butter and sugar until pale and fluffy. Add eggs, one at a time, beating until each is until incorporated, scraping down sides of bowl as needed. Beat in zest and vanilla. Add flour mixture in three batches, alternating with two additions of buttermilk and lemon juice, and beating until just combined after each.
Divide batter evenly among lined cups, filling each three-quarters full. Bake, rotating tins halfway through, until golden brown and a cake tester inserted in centers comes out clean, about 25 minutes. Transfer tins to wire racks to cool completely before removing cupcakes. Cupcakes can be stored overnight at room temperature, or frozen up to 2 months, in airtight containers.
For the meringue frosting
Combine 1 1/2 cups (300 g) sugar with the water and corn syrup in a small saucepan; clip a candy thermometer to side of pan. Bring to a boil over medium heat, stirring occasionally, until sugar dissolves. Continue boiling, without stirring, until syrup reaches 230ºF (110ºC).
Meanwhile, in the bowl of a standing electric mixer fitted with the whisk attachment, whisk egg whites on medium-high speed until soft peaks form. With mixer running, add remaining 2 tablespoons sugar, beating to combine.
As soon as sugar syrup reaches 230ºF (110ºC), remove from heat. With mixer on medium-low speed, pour the syrup down the side of the bowl in a slow, steady stream. Raise speed to medium-high; whisk until mixture is completely cool (test by touching the bottom of the bowl) and stiff (but not dry) peaks form, about 7 minutes. Use immediately.
To assemble the cupcakes
Pipe 1 tbsp (15 ml) lemon curd into the middle of each cupcake. Fill a pastry bag fitted with a large open-star tip with meringue frosting. Pipe frosting onto each cupcake, swirling tip slightly and releasing as you pull up to form a peak. To brown the meringue: Broil for a few seconds in the oven. BE VERY CAREFUL: browning occurs very quickly.You can also use a kitchen torch: Hold the torch 3 to 4 inches (7.5 to 10 cm) over the surface of the meringue and wave it back and forth until the meringue is lightly browned all over.
Serve the cupcakes immediately, or up to a half-day after frosting.
MAKE-AHEAD TIP: You can bake the cupcakes ahead of time, let them cool completely, then freeze them in airtight containers or heavy-duty freezer bags. Thaw the cupcakes in the refrigerator overnight and fill and frost up to a half-day before serving.
Recipe Credit: Adapted from MarthaStewart.com
No buttermilk on hand? You can make some in a pinch: add 3 tsp (15 ml) white vinegar or lemon juice to a measuring cup and then fill with milk up to the 1 cup (250 ml) mark. Stir and then let rest for 5 to 10 minutes. Use as indicated in the recipe.
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