Whisk together your flour and sugar in a medium saucepan over medium heat. Cook for about 2 minutes to toast the flour. Be careful not to burn it!
Slowly add in your milk, whisk to combine, and bring your heat to medium-high.
Bring the mixture to a simmer, whisk continuously until it is very thick and pudding-like. This will take a few minutes. Once it's simmering, cook for one minute more to get the flour taste out.
(Optional) If your mixture has any lumps, place a colander on top of a heat-proof bowl and push your mixture through and into the bowl.
Transfer the base to a heat-proof bowl and place some plastic wrap on top so that it's touching the surface so that a skin does not form. Let it cool in the fridge for a few hours or overnight until it's completely cool and not warm at all.Or you can spread onto a shallow sheet pan, cover with plastic, and freeze for 30 minutes to cool faster. If it's even a little warm it will melt your butter later.
Add your softened butter to the bowl of your stand mixer and whisk on high until very light and fluffy, about 2-3 minutes.
Once the butter is fluffy, place the cooled flour mixture into a piping bag and slowly pipe it into your butter as your whip. Incorporating slowly insures a smooth buttercream.
Add in your vanilla and salt, then mix until everything is creamy. Use it to frost your cooled cake. This buttercream will set quickly, so it's best used immediately.
Frost and decorate your cake any way you want! Watch the video for more ideas.