Place flour and sugar in a large saucepan over medium heat. Cook, stirring constantly, for 30 seconds.

While whisking constantly, slowly pour the milk in (this ensure it's lump free).

As the milk gets hotter, it will start to thicken - stir constantly so the base doesn't catch.

Cook until the mixture thickens in a thick, dolloping custard - see video for texture. TIP: Thicker texture = thicker frosting texture but won't make the frosting dense, it's still fluffy and spreadable but it just makes it "sturdier" with sharper edges when piped.

Remove from heat and scrape into a bowl. Cover with cling wrap, pressing down onto the surface to prevent a skin from forming.

Cool completely (I leave on counter for 20 min or so then refrigerate 30 min to speed up but don't let it get chilled, best at room temp to beat into butter). You can leave in fridge overnight but take it out 1 hour prior to using (to dechill - otherwise it won't mix well with softened butter).

Place butter in a bowl and use either a handheld beater or stand mixer (with whisk attachment) to beat for 3 minutes until it's smooth and changes from yellow to very pale yellow, almost white.

Now start whipping in the Thickening Roux. On speed 5 (medium), start adding the thick roux one heaped tablespoon at a time. Take about 1 minute to add it all.

Once all added, add vanilla and salt, then whip for 2 to 3 minutes until you can see that it is still enough to hold peaks. Then it's ready to use!

Beat in the cocoa powder at the end, just until mixed through.

Use it like any other frosting on cakes and cupcakes - either spread it on with a knife or put in a piping bag. You can pipe sky-high mounds and it will hold its form, as pictured on Vanilla Cupcakes in this post.

See notes for storage / make ahead.