When I am making mmf I only add half the ps called for while still in the mixer.
I then turn it out and knead in as much more powder sugar as the fondant can take.
I find that if you add too much sugar to the mm while still in the mixer it doesn't mix properly and you get lumps of ps.
Add a small amount at first and continue to stir in small amounts until half the bag of PS is left. When the MMs are still very moist the PS wants to just roll around into tiny balls if you dump too much in, but when you add a small amount in the beginning it helps absorb most of the moisture.
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