Next, whisk in the egg and vanilla extract until smooth and well combined. Alternating with the dry, whisk in the sour cream and milk into the wet in this order: dry, milk, dry sour cream, dry until just combined.
Cream the room temperature butter on low until smooth. Stop and scrape down sides of bowl as needed. Mix in the sifted powdered sugar 1/2 cup (60 g) at a time. Once all the sugar has been added, mix in the vanilla extract, salt and heavy cream.