Pre-heat oven to 350F/176C. Line an 8X8 inch cake tin with parchment paper and butter the sides.
Cream together butter, sugar and lemon zest in a bowl. Add the egg and vanilla extract and whisk till combined.
In another bowl, whisk together 1 ¾ cup flour, salt, baking powder and baking soda. Add this to the wet ingredients, along with yogurt. Whisk till just combined.
Toss the blueberries in ¼ cup flour and add them to the cake batter and gently fold them.
Pour the batter into the prepared tin, sprinkle sugar on top and bake for 30-35 minutes till a toothpick inserted into the middle comes out clean. Let it cool for ten minutes before slicing.
Frozen blueberries: Fresh blueberries can be subbed for the same amount of frozen blueberries. Don't thaw them, and bake the cake bars for an extra few minutes till a toothpick inserted into the center comes out clean.
Yogurt: You can substitute the yogurt with ½ cup buttermilk