Take out the butter, eggs, and buttermilk so that they come to room temperature before you start, and while they come to room temperature zest the lemons and then juice the lemons. Preheat the oven to 350 degrees. Grease and flour 2 (8 1/2 by 4 1/4 by 2 1/2-inch) loaf pans.
Cream the butter and 2 cups of the granulated sugar with an electric mixer until light and fluffy, about five minutes. Then add the eggs, one by one, mixing each one in at medium speed, and add the lemon zest.
Sift together the flour, baking powder, baking soda, and salt in a separate bowl. In another separate bowl, combine the buttermilk with 1/4 cup of the lemon juice and the vanilla. Add the flour and buttermilk mixtures alternately to the bowl, beginning and ending with the flour mixture. Divide the batter evenly between the two pans, smooth the tops, and bake for 45 minutes to an hour- until a toothpick inserted in the center comes out clean.
Combine the last 1/2 cup of granulated sugar with the other 1/2 cup of lemon juice in a small saucepan and cook over low heat until the sugar dissolves. When the cakes are done, allow them to cool for 10 minutes. Then remove them from the pans and set them on a rack set over a tray/sheet pan. Spoon the lemon/sugar mixture over them.
Allow the cakes to cool completely.
Whisk together the 3 1/2 teaspoons of lemon juice with the 2 cups of confectioners sugar in a bowl until the sugar is completely dissolved.
Drizzle the glaze over the cakes.