Position a rack in the center of the oven and preheat to 375°F Lightly spray a 12-cup muffin pan with the cooking spray.
To make the cakes, sift together the flour, corn meal, sugar, Clabber Girl baking powder and salt into a medium-size bowl. In a separate bowl, whisk together the oil, eggs and yogurt. Make a well in the center of the dry ingredients, add the egg mixture, and stir just until blended. Do not overbeat. Let sit for the polenta to absorb the liquids while preparing the muffin tin.
To prepare the tin, in each muffin cup place 1 teaspoon of the butter and 1 teaspoon of the sugar, then add 2 strawberry halves. Sprinkle evenly with the diced rhubarb. Bake until the mixture in each cup is melted and bubbly, about 8 minutes. Remove the pan from the oven and divide the batter evenly among the prepared muffin cups, filling them as full as possible.
Return muffin tin to the oven and bake until the center of a muffin springs back when pressed gently with your finger, about 18 - 20 minutes. Immediately invert and unmold the cakes onto a wire cake rack. Cool briefly and serve. These cakes are best the day they are made or kept in an airtight container for up to three days.