The cooked custard buttercream is similar to a cooked roux buttercream. . .
I have not made a cooked custard buttercream, BUT I have made and love a cooked roux (pudding) buttercream and love it. It is not too sweet, looks great, doesn's seem to get a crust. Also, you will get a more white frosting with the recipe below as it is not yellow tinted due to the egg yolks--and it is pretty simple; works for me.
I do like the idea of the extra richness from the egg yolks though! Saving that link and recipe.
Cooked Roux (pudding) buttercream
1 cup milk
3 tablespoons flour
1 cup white sugar
1 cup butter
directions
Cook milk and flour until thick, stirring constantly. Remove from heat and cool to *below room temperature*, stirring occasionally. Make sure it is down below room temp as you will not want to melt the butter when you proceed to the next step.
Cream sugar and butter until real fluffy. Add flour mixture, very slowly, 1 tablespoon at a time. Beat well after each addition.