Preheat the oven to 350ºF. Prepare a 8 or 9-inch square baking pan with non-stick cooking spray.
Using a mixer, cream the softened butter, 1 cup of sugar and the lemon zest until light and fluffy.
Add in the eggs and vanilla and mix until combined.
In another bowl, whisk together the flour, baking powder and salt. (You can sift the flour, into another bowl, if you want even more of a fluffier texture)
Gradually add in the flour mixture to the wet batter a little at a time, alternating with the buttermilk until incorporated. Fold in the blueberries.
Spread batter into pan and sprinkle the batter with a tablespoon of sugar.
Bake for 40 - 45 minutes for an 8 inch pan and 30-35 minutes for a 9 inch pan. Check with a toothpick for doneness. If cake appears to getting brown before the timer goes off, add a layer of kitchen foil. Halfway through baking, you can place a layer of kitchen foil over the top of the cake to prevent over-browning,
Return pan to oven for a couple of more minutes, if necessary.
Let cool at least 15 minutes before serving.
Enjoy the blueberry explosion!