Just suggestions and ideas ..
smittenkitchen - dense/dry - gluten developed/overbeaten
* with butter cake, once the fat a has been creamed with the sugar and the egss added, the flour/liquid should be added by hand. If you beat them in using a machine, you run the risk of gluten developing. This causes a dense crumb with a dry mouthfeel.
sweetpolita - collapsed - over aerated/overbeaten
* beat the egg whites until just to stiff peak - watch carefully and stop immediately.. Beating egg white unravels the protein so that it forms a structure that can trap air. If the egg white is over beaten, the structure locks firmly into position, and when it cools, the structure collapses.
dorie - didn't rise - under creamed
* beating fat with sugar requires 5-8 minutes not 3 minutes. The egg is added with the liquid, so it's not going to contribute much to the aeration of the batter.
It appears that the the reason that you are machine mixing the flour into the batter is so that the gluten from the cake flour provides the structure for the aeration ..
I would be tempted to beat the egg white to peak. Cream the fat with the sugar - fold in the flour and the liquid. . then fold the egg white back into the batter.