Preheat oven to 375°F Coat an 8-inch-square baking dish (or other 2-quart ovenproof dish) with cooking spray.
Combine cornmeal, buttermilk, maple syrup, butter and salt in a large saucepan. Cook over medium-high heat, whisking constantly, until the mixture bubbles and thickens. Remove from the heat. Whisk egg yolks in a large bowl. Whisking constantly so the eggs won’t scramble, add 1/2 cup of the hot cornmeal mixture until completely combined. Scrape in the remaining cornmeal mixture and whisk until smooth. Whisk in flour and vanilla until smooth.
Beat egg whites in another large bowl with an electric mixer on high speed until soft peaks form. Gradually sprinkle in sugar, continuing to beat until soft glossy peaks form. Fold the whites into the cornmeal mixture. Fold in 2 cups raspberries. Spoon the batter into the prepared dish.
Bake until puffed, golden brown on top and the center is set, 35 to 40 minutes. The spoonbread will be slightly wobbly and soft when it is completely cooked. Let cool for 20 minutes. Serve warm, topped with some of the remaining raspberries and yogurt, if using.