Then cream butter, powdered sugar, and vanilla together into a fluffy buttercream.
Add the room temperature salted caramel sauce and continue to blend it with the sugar and butter. (If your caramel sauce has gotten too firm, warm it up just enough to pour it into the frosting).
Add flour and continue to mix. (You can also use more powdered sugar instead of the flour).
Add 1-2 teaspoons of plant-based milk if necessary to get the right texture.
Notes
Depending on the type of vegan margarine that you use, you may need to add a small amount of plant milk to the frosting to get the right consistency.
You may need to put the frosting in the fridge for about 10 minutes before piping if the caramel was too warm when you added it to the frosting.
If your frosting is too thin, add a few more tbsp of powdered sugar.
Note: The trick to making this frosting come out well is keeping the ingredients at room temperature. The butter should be just soft enough to whip into the frosting and the caramel sauce cannot be too warm or it will melt your frosting.