In the work bowl of a stand mixer, combine egg whites, brown sugar, lemon juice, vanilla extract, cream of tartar, and salt. Set bowl over a pan of simmering water, being careful not to let bottom of bowl touch water. Whisk constantly, until mixture registers 140° on an instant-read thermometer.
Transfer bowl to mixer, and beat on high speed until tripled in volume and glossy, about 10 minutes. Place mixture in a pastry bag, and use immediately.
Recipe by Victoria at https://victoriamag.com/blueberry-cupcakes-with-lemon-curd-filling-recipe/