Sourdough Frosting

In a small saucepan over medium-low heat, whisk the sugar, salt, whole milk, and sourdough discard (or active starter).
Bring the mixture to a simmer, whisking constantly, until it thickens into a roux (pudding-consistency) and leaves trails of your whisk in the saucepan.
Remove from the heat and scrape into a mixing bowl to cool to room temperature. To prevent a film from forming, press a piece of plastic wrap onto the surface.
200 g Whole Milk, 200 g Granulated Sugar, ¼ tsp Kosher Salt, 80 g Sourdough Discard
In the bowl of a stand mixer fitted with the whisk attachment (or using an electric hand mixer in a large mixing bowl), whip the softened butter until it's light and fluffy. Scrape the sides of the bowl as needed.
226 g Unsalted Butter
While mixing on medium speed, spoon the cooled sourdough roux mixture into the whipped butter, one large spoonful at a time until it's fully mixed. This can take several minutes.
Whisk until the frosting is smooth, light, and fluffy. Add the vanilla extract at the end of mixing. Taste and adjust the flavor with more vanilla, powdered sugar if you want it sweeter, or a bit of milk if it's too thick.
1 tsp Vanilla Extract