Weekend Plum Cake

Stir yeast and warm water together in a large bowl, let stand for 5 minutes or until foamy. Add flour, 1/2 cup sugar, salt, sour cream, egg, lemon zest and vanilla to yeast mixture, and beat on medium speed until combined. Incorporate 1/2 cup butter 1 tablespoon at a time, and beat for an additional 5 minutes until smooth. Scrape sides of the bowl and cover with a kitchen towel. Let rise at room temperature for 1 1/2 - 2 hours. Spread 2 tablespoons butter in the bottom of a 9 inch circle cake pan, and sprinkle with 1/3 cup sugar. Arrange plum halves cut side down and arrange in one layer on top of the butter and sugar. Spread dough evenly over the plums. Cover with buttered plastic wrap, then kitchen towel on top and let rise at room temperature for 1 1/2 hours. Preheat oven to 375F. Bake cake for 30-35 minutes (test by inserting a wooden toothpick into the center of the cake - the cake is done when inserted toothpick comes out clean). Cool for 5-10 minutes. Turn cake upside down onto serving plate and let stand until completely cooled. Serve with a big spoonful of vanilla ice cream!