I would love to know what recipe you are using but that is entirely up to you if you wanted to share it.
You said :
"So in my last test, I cream the butter for 20 sec first before add the dry, but still the batter ended up with lumps...."
From all the cakes I have made 1) 20 seconds is not a long time to cream your butter
2) was there liquid in this because you did not mention (I assume there was) ?
3) when combining the dry ingredients into the creamed butter do you simultaneously add the liquid a little at a time ?
Lucy, I am just trying to troubleshoot your problem with the lumps.......the fact that you would like to perfect your recipe is very commendable. Le Cordon Bleu ? I have the dessert technique book but do not see any cake recipes....it would be good to know your recipe and the steps.
There are so many talented members here, maybe they will chime in.....