For the Lemon Sugar:

1. In a bowl, mix together the sugar and the zest from two lemons.
2. Using your hands, rub the zest into the sugar so that the natural lemon oils flavor and perfume the sugar.

For the Batter:

1. Preheat the oven to 350°F and liberally grease a loaf pan.
2. To a stand mixer, add 1 cup of the lemon sugar (reserve the other ½ cup for the topping) and the butter and mix until light and fluffy, about 2 minutes.
3. Add in the sour cream, lemon juice, and vanilla and mix to combine.
4. Add in the eggs, one at a time, and mix after each egg is added. Scrape down the sides of the bowl to ensure that all the ingredients are incorporated.
5. In a medium bowl, whisk together 1 cup of the flour, salt, and baking soda. Add to the wet ingredients and mix until just combined.
6. To the same bowl, add the remaining ½ cup of flour and the blueberries. Stir to coat the blueberries in the flour. Using a spatula, gently fold the blueberries and residual flour into the batter – do this by hand so that you don’t break up the berries too much.
7. Pour the batter into the prepared pan and smooth out the top.
8. Sprinkle the reserved ½ cup of lemon sugar evenly over the top of the batter.
9. Bake for 50-60 minutes, first checking around 45 minutes, until the edges are golden and the center is set (insert a toothpick and if it comes out clean, the cake is done).
10. Cool in the pan for 20 minutes and then carefully invert the cake to remove from the pan and set it on a wire rack to cool completely. Serve warm or at room temperature.