In a small bowl stir together flour, cornmeal, baking powder, lemon peel, salt, and nutmeg. In a medium bowl beat the 1/4 cup butter with an electric mixer on medium to high speed for 30 seconds. Add sugar and vanilla; beat until combined. Add eggs, one at a time, beating well after each addition. Add sour cream; beat until combined. Gently stir in cornmeal mixture.
Generously butter an 8- to 9-inch cast-iron skillet or casserole or two individual 10- to 12-ounce cast-iron skillets with the 1 tablespoon butter. Add plums to skillet(s). Spoon batter over plums; spread evenly.
For a charcoal grill, arrange medium-hot coals around the edges of the grill, leaving center of grill with no coals. Test for medium heat above the center of the grill. Place skillet(s) in center of grill rack (not over the coals). Cover and grill until a wooden toothpick inserted near the center(s) comes out clean. Allow 25 to 30 minutes for large skillet or about 20 minutes for small skillets. (For a gas grill, preheat grill. Adjust for indirect cooking. Place skillet(s) on unheated side of grill. Grill as above.)
Remove from grill; let cool for 30 minutes before serving. If desired, serve warm with Whipped Cream.
In a medium bowl combine 1/2 cup whipping cream, 1 tablespoon sour cream, and 1 tablespoon powdered sugar. Beat with an electric mixer or wire whisk until soft peaks form (tips curl).