Flour Buttercream.

Directions

1. 1
   Heat milk and heavy cream in a medium-sized sauce pan over medium head.
2. 2
   Whisk in flour. Keep whisking vigorously until mixture is very thick. 5-10 minutes. Mixture should be a smoothe and lump-free paste.
3. 3
   Cover pot loosely with plastic wrap and put in fridge for at lest 20 minutes, or until cooled.
4. 4
   With an electric mixer (it is best to use a stand mixer if you have one) beat the margarine, butter, and sugar for 20 minutes on a medium-high speed. It's very important to cream this long.
5. 5
   Add in milk & flour thickening. Beat the mixture on medium-high for another 15 minutes.
6. 6
   Add in vanilla (I usually use about a tbs) and beat for another minute.