Preheat oven to 350°F and grease or line a loaf pan with parchment paper (the size I've used is 8.5x4.5 inches - you can use a slightly bigger one too, it just may not be as "tall").
Stir together flour, baking powder, salt, and poppy seeds. Set aside.
Add lemon zest and sugar in a mixing bowl, rubbing the mixture between your fingers until a coarse sand-like mix forms. Using an electric hand mixer, cream in room temperature butter. Add lemon juice, vanilla extract, and lemon extract until combined. Add eggs, one at a time, mixing between each addition.
Alternate adding the dry ingredients and the wet (vegetable oil and sour cream). Starting and ending with the dry ingredients (I do 3 dry additions, and 2 wet).
Bake for 52-55 minutes until golden brown and a toothpick inserted comes out clean. If you use a different size pan, cooking time may vary slightly, so be sure to check it close to the end. Let cool in pan for 5 minutes, remove and cool completely on a cooling rack.
Icing: Using an electric mixer, combine lemon juice, vanilla bean, butter, and powdered sugar until smooth. Once loaf is completely cooled, spread on top.