Step 1: In a medium saucepan, whisk together the granulated sugar and all-purpose flour until well combined and lump-free.
Step 2: Gradually whisk in the whole milk until the mixture is smooth. Place the saucepan over medium heat.
Step 3: Cook the mixture, stirring constantly with a whisk, until it thickens to the consistency of a thick pudding or custard. This usually takes about 5-7 minutes. Ensure no lumps form.
Step 4: Remove the pan from the heat and stir in the vanilla extract.
Step 5: Transfer the cooked mixture to a shallow bowl or plate. Cover the surface directly with plastic wrap to prevent a skin from forming, and let it cool completely to room temperature. This step is critical; if the mixture is not cool, it will melt the butter.
Step 6: In a large mixing bowl, using an electric mixer with the paddle attachment (or a stand mixer), cream the softened unsalted butter on medium-high speed until it’s very light, fluffy, and pale in color. This should take 3-5 minutes.
Step 7: With the mixer on low speed, begin adding the cooled flour mixture to the creamed butter, one tablespoon at a time. This gradual addition helps prevent curdling.
Step 8: Once all the flour mixture has been added, increase the mixer speed to medium-high and beat for another 5-7 minutes. The frosting will initially look curdled but will eventually transform into a light, smooth, and incredibly fluffy frosting. Continue beating until it is perfectly smooth and has increased significantly in volume.
Step 9: If desired, add a pinch of salt to balance the flavors. Taste and adjust sweetness if necessary, though this recipe is designed to be less sweet.
Step 10: Your fluffy less sweet ermine frosting is now ready to use to frost your favorite cakes, cupcakes, or other baked goods!