Preheat the oven to 350 and butter a 9-inch cake pan.
Whisk together the flour, baking powder, and salt in a medium bowl until combined. Set aside.
Using a freestanding electric mixer with a paddle attachment (or a handheld whisk), cream together the butter and sugar on medium-high speed until very light and fluffy, at least 4 minutes. Lower the speed of your mixer and add the eggs one at a time, beating well after each addition.
Using a rubber spatula, scrape down the sides of the bowl and add the lemon juice, lemon zest, and rosemary. Continue beating until the batter on low speed until it is smooth and combined. Continue to have your mixer on slow speed. Slowly pour in half the buttermilk. Beat until just mixed, then add half the flour/baking powder/salt mixture and continue mixing until the flour is just incorporated.
Scrape down the sides of the bowl and repeat this process until all the buttermilk and flour/baking powder/salt mixture have been added. Beat until smooth, but be careful not to overbeat or your cake will be tough and fudgey as opposed to light and airy.
Pour the batter into the greased pan, smoothing out the top with a rubber spatula. Bake cake in the preheated oven for 40 minutes until the edges are golden and begin to pull away from the sides of the pan. A toothpick inserted in the center should come out clean. Place the cake on a wire rack to cool completely.