In the bowl of a stand mixer, beat together butter and sugar until light and fluffy, about 3 to 5 minutes. Add egg, and mix until combined. Add vanilla, lemon zest, and lemon juice, and mix until combined. Working in batches, add one-third of flour mixture. Beat until combined, scraping down sides of bowl. Add ¼ cup buttermilk; mix completely, scraping bowl. Add second batch of one-third of flour mixture. Scrape a third time, and add remaining ¼ cup buttermilk. Finish with flour mixture, scraping down sides of bowl, if necessary, until fully combined.