In a small saucepan, bring sugar and ¼ cup (60 grams) water to a boil over medium heat, stirring occasionally. Cook until mixture registers 238°F (114°C) on an instant read thermometer.
Meanwhile, in the bowl of a stand mixer with the whisk attachment, beat egg whites at high speed until soft peaks form. With mixer on medium speed, add sugar syrup in a slow, steady stream until combined. Increase mixer speed to high, and beat until bowl is cooled to room temperature.
With mixer on medium speed, add butter, a few tablespoons at a time. Add vanilla, and increase mixer speed to high; beat until well combined. Use immediately.
Recipe by Bake from Scratch at https://bakefromscratch.com/fraisier-layer-cake/