For the batter:
• 1 to 1¼ cup chopped rhubarb (about 1 large stalk)
• 1 cup of sourdough starter of middling ripeness (i.e. no more than four or five days away from its last feeding—if it's super compact, sour, and ill-fed, use less)
• 1 cup whole wheat flour
• ¾ teaspoon baking soda
• ¼ tsp salt
• 1⁄3 cup yogurt
• 3 tablespoons vegetable oil
• 1⁄3 cup sugar
• 1 egg
• 1 cup lightly cooked, lightly sweetened rhubarb
For the topping:
• 3 tablespoons butter
• ½ cup brown sugar
• 1 teaspoon flour
• a few drops vanilla extract
Preheat the oven to 350 F. Lightly grease a nine-inch round cake pan (nine-inch square is fine, too, just be sure to check the coffee cake a few minutes before the time listed here, and adjust baking time according to doneness).
Prepare the rhubarb:
Rinse and chop rhubarb coarsely and add to saucepan with a tablespoon of water and a few tablespoons of sugar. Bring to simmer over low heat. Taste and adjust the sugar. You want it to be edible but sour. If you wind up with a lot of liquid, drain some of it off before adding to the coffee cake batter.
Mix the batter:
Mix all the batter ingredients except for the rhubarb together until just combined. If your starter is very stiff, you may need to add a few tablespoons more yogurt to the batter to moisten it up (it should still be very thick). Add the rhubarb and stir. Scrape into cake pan, pushing batter out evenly.
Make the topping:
Cream together the butter, brown sugar, flour, and vanilla extract for the topping. Sprinkle over the batter. Bake for 30 to 40 minutes.
To read Heather's post about using sourdough starter in more than just bread, click here.