Preheat the oven to 400 degrees F (200 degrees C). Grease a 12 cup muffin tin, or line with paper liners.
In a large bowl, mix together the yogurt, oil, lemon juice, egg whites, and, if using, lemon extract. In a separate bowl, stir together the flour, 3/4 cup sugar, baking powder, salt, and lemon zest. Add the wet ingredients to the dry, and mix until just blended. Gently stir in the frozen raspberries. Spoon batter evenly into the prepared muffin cups. Sprinkle remaining sugar over the tops for decoration, if desired.
Bake for 15 to 17 minutes in the preheated oven, or until the top springs back when lightly touched. Cool muffins in the tin on a wire rack.