Preheat the oven to 350°F. Grease a 9" x 5" loaf pan (I recommend a metal pan that's light colored) with nonstick spray; set aside.

Using a stand mixer fitted with the paddle attachment on medium speed, beat the butter, sugar, and lemon zest together until light and fluffy, about 3 minutes, scraping down the bowl halfway through.

Add sour cream and mix just to combine.

Add the eggs one at a time, followed by the egg yolk, mixing well after each addition. Scrape down the sides of the bowl as needed. 

Next add the lemon juice and vanilla extract, and mix just to combine. The mixture will have a curdled appearance.

In a medium bowl, whisk together the flour, salt, and baking powder. 

With the mixer on low speed, slowly add the dry ingredients to the wet. Mix just until evenly combined. Do not over mix.

Transfer the batter to the prepared loaf pan and gently smooth the top.

Bake for about 70 minutes (1 hour 10 minutes). The top center of the pound cake should be domed, cracked, lightly browned, and very lightly firm (not overly jiggly) to the touch. I recommend that you start checking the pound cake for doneness at 55 minutes, as oven temperatures can vary - you don't want to over bake this cake.

Let the cake cool in the pan for 10 minutes, then turn it out onto a cooling rack and let cool completely before adding the glaze. If the cake is still warm when you add the glaze, the glaze will get melty and thin out, and run off the cake.

In a small bowl, whisk together all of the ingredients until smooth. It should be thickened, yet slowly pourable. If too thin, add more powdered sugar. If too thick, add just a little more lemon juice. Drizzle over the cooled pound cake. 

Allow the glaze to set completely before slicing, at least 15 minutes.