Cook sweetened flour paste. If you plan to sieve your flour paste, place your sieve atop a shallow tray or plate for easy access. Otherwise, just have your tray out and ready.

Get a timer ready. Set the timer for one minute for every cup of milk/cream.

In a pan, whisk together the flour and sugar. Pour in the cold milk and heavy cream. Whisk until you no longer see lumps of flour.

Place the pan on medium-low heat. Cook the flour mixture, whisking continuously and scraping the sides. It will gradually start to thicken into a loose pudding.

Once you see large bubbles pop in the center, start your timer and continue to whisk vigorously.

Sieve (optional) and cool sweetened flour paste. When the timer is up, use a spatula to scrape the hot paste onto your tray.

Spray a piece of plastic wrap with oil and press it directly onto the surface of the paste. Let this sit at room temperature to cool.

Aerate butter. Add the softened butter to the bowl of your stand mixer. Using the whisk attachment, beat the butter on high speed until it's paler in color and slightly more voluminous.

Add the vanilla extract and pinch of salt and mix on high speed for 2 minutes.

Add cooled flour paste in 4 to 5 additions. Mix on high speed for a few seconds before adding the next.

Adjust buttercream for texture. Switch to the paddle attachment for smoothing. Run the mixer on low speed for a few minutes until most of the air pockets are smoothed out.