Preheat oven to 350°F (175°C).
Grease a 6-inch round cake pan and line the bottom with parchment paper.
In a small bowl, whisk together 3/4 cup flour, baking powder, baking soda, and salt. Set aside.
In a medium bowl, use a whisk or electric mixer to cream softened butter and granulated sugar until light and fluffy (about 2 minutes).
Beat in the egg until fully combined.
Mix in vanilla extract and lemon zest (if using).
Add half the dry ingredients to the butter mixture, mixing gently until just combined.
Stir in buttermilk, then add the remaining dry ingredients. Mix until smooth, but do not overmix.
In a small bowl, toss blueberries with 1 tsp flour to prevent sinking.
Gently fold blueberries into the batter using a spatula.
Pour batter into the prepared pan, smoothing the top.
Sprinkle coarse sparkling sugar evenly over the batter for a crunchy topping.
Bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean and the top is golden.
Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
Slice and serve warm or at room temperature.
Pair with coffee, tea, or espresso (optionally with a blueberry flavor shot) for the full Dunkin' experience.