Step 1

Preheat oven to 350°. Butter an 8½x4½" loaf pan and line with parchment paper, leaving a generous overhang on long sides. Whisk flour, salt, baking powder, and baking soda in a medium bowl; set aside.

Step 2

Place granulated sugar, lavender, and lemon zest in a food processor. Scrape in seeds from vanilla bean; save pod for another use. Pulse until lavender is finely chopped; set 1 Tbsp. lavender sugar aside for topping.

Step 3

Using an electric mixer on medium-high speed, beat remaining lavender sugar and ½ cup butter in a large bowl until very light and fluffy, 5–7 minutes (don’t shortchange yourself here; the long beating time aerates the cake and yields the finest texture). Add eggs one at a time, beating to blend after each addition and scraping down sides and bottom of bowl as needed.

Step 4

Combine buttermilk and lemon juice in a small bowl. Reduce speed to low and add dry ingredients to lavender sugar mixture in 3 additions, alternating with buttermilk mixture in 2 additions, beginning and ending with dry ingredients. Stop mixer just before all dry ingredients are incorporated and finish mixing by hand, a guarantee against overmixing the batter, which can cause the cake to be tough. Scrape batter into prepared pan and smooth top with a spatula.

Step 5

Bake cake until a tester inserted into the center comes out clean, 55–65 minutes. Transfer pan to a wire rack; let cake cool in pan 20 minutes before turning out and peeling off parchment. Let cool completely.

Step 6

Do Ahead: Lavender sugar can be made 1 month ahead; store airtight at room temperature. Cake can be baked 2 days ahead; store tightly wrapped at room temperature.

Step 7

Whisk powdered sugar and buttermilk in a medium bowl. Pour over cooled cake, letting excess drip over sides. Using a lemon zester, zest lemon into strips into a small bowl; add 1 Tbsp. reserved lavender sugar and toss to coat. Top cake with sugared lemon zest. Let sit until glaze is set, about 30 minutes.

Step 8

Do Ahead: Glazed cake (without zest) can be made 1 day ahead; store under a cake dome (or an overturned bowl) at room temperature. Top with sugared lemon zest just before serving.