* Mine did not double. It barely did anything. I had a warm kitchen, a plastic wrap and a towel atop the bowl, and a tea towel wrapped around the bowl. So I balked. Balked! I baked it without the plums the first time: just a sprinkle of cinnamon. It puffed marvelously in the oven and it tasted like a moist, buttery coffee cake, without being heavy. Second time around (the next morning): not much action again. Since my yeast-water mixture foamed both times, and the cake baked up fine on the non-plum version, I figured I'd be okay the second go-round. It was perfect.
** I had extra, especially since I decided the plum "heels" weren't cake-worthy. Grub and I ate them.
*** A little more rise noted here on the plum version. But still it still did not double until I got it into the oven and poof!, like magic.
**** Yeah, easier said than done. I gave it exactly 5 minutes, flipped the pan over, and the kuchen wouldn't budge. After waiting a little longer and thwacking the pan a few times, then it released. A few plum slices dislodged from their bed of cake crumb, which were easily tucked back into place. I almost lined the pan with parchment, because I feared cake release would be difficult, but held back because I thought it might affect how the plums cooked into the cake.