Preheat oven to 350 degrees F (177 degrees C) and place rack in center of oven. Butter or spray with a pan spray, a 9 x 5 x 3 inch (23 x 13 x 8 cm) loaf pan. Line the bottom of the pan with parchment paper and butter or spray the paper.
In a large bowl, sift together twice, the flour, baking powder, and salt.
In the bowl of your electric mixer, or with a hand mixer, beat the butter until creamy and smooth. Gradually add the sugar, beating continuously on medium-high speed until light and fluffy (this will take about 5 minutes). Scrape down the sides of the bowl as needed. After about five minutes the batter should be light in color and fluffy in texture. Then add the eggs, one at a time, mixing well after each addition. Scrape down the sides of the bowl as needed. You will notice that the batter will look curdled. Don't worry as the batter will come together again after you add the flour mixture. Add the vanilla and lemon zest, if using, and beat until incorporated.