Grease and flour one large loaf pan (12x5 inches and 3 ½ inches high or 30x13 cm and 9 cm high) or two small pans. Preheat oven to 350°F (170°C).
In a large mixing bowl, beat butter and sugar until light and fluffy. Add lemon zest and beat in eggs, one at a time, until incorporated and scraping bowl between additions.
In a measuring cup, add lemon juice and vanilla extract to the buttermilk.
In another large bowl, whisk together flour, baking powder and salt.
Alternately add flour mixture and buttermilk mixture to the butter mixture, beating until incorporated and scraping bowl between additions.
Pour batter into prepared pan(s) and bake for 55 minutes*, or until toothpick inserted near the center comes out clean.
Cool in pan on a wire rack for 10 minutes. Run a long, sharp knife along the inside of the pan to release cake from sides. Tip cake out of pan and cool completely on a wire rack.
Whisk the glaze ingredients together and pour over top of warm or cooled cake. Store covered at room temperature for up to three days.
Notes
*If using small loaf pans, decrease baking time by 15 mins.UPDATE 1/2023: Due to readers' feedback that their cakes were sinking, I did some research and found that the lemon juice was likely the culprit. Although the cakes tasted great, the acid in the lemon juice was reacting with the baking powder and altering the overall consistency of the cake. This has not happened in our kitchen and I love the punchy lemon flavor. However, to be on the safe side, I have reduced the lemon juice in the cake batter recipe above from the juice of 2 lemons to the juice of ½ lemon.