Preheat oven to 350 degrees. Butter and flour two 8 1/2-by-4 1/2-inch (6-cup) loaf pans; set aside. In a medium bowl, whisk together flour, salt, baking soda, and lemon zest; set aside.

In a large bowl, using an electric mixer on high speed, beat butter and granulated sugar until light and fluffy. Add eggs one at a time, beating well after each addition; mix in vanilla. With mixer on low, alternately add flour mixture in three parts, and sour cream in two, beginning and ending with flour mixture.

Divide batter between prepared pans; smooth tops. Bake until a toothpick inserted in center of cakes comes out clean, 55 to 65 minutes (tent loosely with foil if browning too quickly). Meanwhile, make Candied Lemon Slices and Lemon Syrup.

Remove cakes from oven; while they're still in pans, use a wooden skewer or toothpick to poke several holes in tops. Setting aside 1/4 cup Lemon Syrup, pour remainder over cakes. Let cakes cool completely in pans, about 2 hours. Remove from pans, and place, right side up, on a rack set over a rimmed baking sheet. Brush cakes all over with reserved syrup.

In a small bowl, whisk confectioners' sugar with 2 tablespoons lemon juice; add up to 2 more tablespoons juice until pourable but thick.

Pour lemon glaze over cakes (still on cooling rack), letting it drip down the sides. Let set, about 30 minutes. Garnish with Candied Lemon Slices.