Preheat oven to 350°F. Line a cupcake pan with 8 liners. 

In a small bowl, combine salt, baking powder and flour. Set aside.

In a large bowl or stand mixer, cream together butter and sugar until light and fluffy. Mix in egg, lemon zest, lemon juice and vanilla until smooth. Scrape bowl as needed. Alternate spoonfuls of flour and sour cream, continuing to mix until batter is smooth and no streaks remain. 

Divide batter between prepared baking cups, using a large cookie scoop or filling roughly 2/3 full. 

Bake for 18-20 minutes or until cooked through and only a few moist crumbs cling to a toothpick. Cool on a wire rack. 

Using a sharp knife or cupcake corer, remove the center of each cupcake, being sure to leave the bottom of the cupcake intact. Fill with lemon curd, using a spoon or pastry bag. Chill until ready to frost. 

In a heatproof bowl set over simmering water, whisk together egg whites, sugar and cream of tartar, until egg whites are warm to the touch and sugar is dissolved. Mixture should be frothy. Remove from heat. Beat, using a hand mixer or stand mixer with whisk attachment, until glossy and stiff peaks form. Add vanilla in during last 30 seconds. Transfer to a large piping bag fit with desired piping tip. Pipe on chilled cupcakes. Lightly char frosting using a kitchen torch.