In a large bowl, whisk together milk, oil, vinegar, and mix.
Divide batter evenly into pans, filing cavities approximately 2⁄3 full.
In two batches, microwave on high for 2 min 30 sec. Rest in pan 5 min; invert onto wire rack to cool. Or bake in preheated 350° F oven, 15–18 min. Cool in pan 10 min; unmold.
When completely cool, spread on icing, if desired.