Step 1: In a medium saucepan, whisk together the all-purpose flour and 1/2 cup (100g) of the granulated sugar until well combined and no lumps remain.
Step 2: Gradually whisk in the whole milk until the mixture is completely smooth.
Step 3: Place the saucepan over medium heat. Cook, whisking constantly, until the mixture thickens significantly, resembling a very thick pudding or gravy. This usually takes about 5-7 minutes. Ensure no lumps form.
Step 4: Remove the saucepan from the heat. Stir in the salt. Pour the hot flour mixture into a shallow dish (like a pie plate) or a bowl. Cover the surface directly with plastic wrap to prevent a skin from forming as it cools.
Step 5: Allow the flour mixture to cool completely to room temperature. This is crucial! It can take several hours on the counter or about 1-2 hours in the refrigerator. It must be cool to the touch, not even slightly warm.
Step 6: In the bowl of a stand mixer fitted with the paddle attachment (or using a hand mixer), cream together the softened unsalted butter and the remaining 1/2 cup (100g) of granulated sugar on medium-high speed. Beat until light, fluffy, and pale in color, about 3-5 minutes. Scrape down the sides of the bowl frequently.
Step 7: Switch to the whisk attachment (if using a stand mixer). With the mixer on low speed, gradually add the cooled flour paste, one spoonful at a time, to the butter mixture. Continue mixing until all the paste is incorporated. The mixture will likely look curdled or separated at this stage; this is normal.
Step 8: Increase the mixer speed to medium-high. Whip the frosting for 5-10 minutes, or until it completely transforms into a light, airy, and fluffy consistency. It should be smooth and voluminous. Be patient; this step is key to the Ermine frosting's texture.
Step 9: Beat in the pure vanilla extract until just combined. If desired, add any colorings or additional flavorings at this stage.
Step 10: Use immediately or store as directed. Once prepared, this fluffy less sweet ermine frosting can be stored in an airtight container in the refrigerator for up to a week. I highly recommend using durable Basics Glass Food Storage containers for optimal freshness and visibility. Bring to room temperature and re-whip briefly before using.