Preheat your oven to 350°F (180°C) for small loaves or 325°F (165°C) for a large loaf. Line your loaf pan(s) with parchment paper.
In a mixing bowl, combine flour, baking powder, and salt. Set aside.
In a separate large bowl, cream together the butter and sugar until light and fluffy.
Gradually add the eggs, one at a time, beating well in between each addition. Mix in the vanilla extract and lemon zest.
Alternate adding the dry ingredients and buttermilk to the wet mixture, mixing until just combined to create a smooth batter.
Divide the batter evenly into the prepared loaf pan(s).
Bake for 25-30 minutes for small loaves or about an hour for a large loaf, until golden brown. Use a skewer to check for doneness.
Let the loaves cool in the pan for 15 minutes, then transfer them to a wire rack to cool completely.
To make the basil sugar, process sugar, basil leaves, and additional lemon zest until fine.
Sprinkle the basil sugar over the cooled cakes before serving.