I am not quite sure by what you mean by a "Buttercreme " Frosting...but here goes the favourite one for the Red Velvet Cake, although I much prefer to use Betty Crocker's Red Devil Food Cake from the 1956 cookbook.
2 T cornstarch
1 C whole milk
1/2 C butter- softened
1/2 C shortening-room temp
1C sugar-granulated
1 t vanilla extract
1/4 t almond extract
With a whisk, in a saucepan, blend together the milk and cornstarch. Over medium heat, stirring all the while, cook until it comes to the bubbling stage and is thick like cornstarch pudding. Remove from the heat, stir in the extracts and pour into another bowl and cover the top with plastic wrap ( to prevent a skin from forming) and refridgerate till cold.
In a mixing bowl, cream together the shortening and the butter till well blended. Gradually, add the sugar till well combined. What I do is add it one spoonful at a time and beat a good minute after each addition. You will see how this one cup of butter literally quadruples in size. And it will also still feel a wee bit grainy but not to worry as that will go away when you add the milk mixture.
Now before you add the milk and cornstarch mixture, give that a good stir and then add to the creamed mixture. Then beat, on a medium speed for ten yes ten minutes till all is very light and fluffy like whip cream and it will no longer be grainy. Frost your cake...and this makes enough to fill and frost an 8" layer cake.
Some notes....my mother always used butter and whole milk...no substitutes. And make sure that the "pudding" is cold and also your cake! My mother's original recipe, gasp!, called for 2 egg whites stiffly beaten. A wonderful southern cook, Char, told me how to switch that with the "pudding" to get basically the same icing. Although today, with the pasterized egg whites, I could go back to it, I still make it the "pudding" way. Hope this helps.