Preheat oven to 350° and then prepare a 9 x 5-inch loaf pan with butter or non-stick spray. Dust pan with cake flour (see video in post.)
In a large mixing bowl, combine the flour, baking powder, and salt. Set aside.
Next, use a food processor to combine the sugar and lemon zest.
Using a stand mixer, combine the butter and the sugar mixture and then cream together until light and fluffy. Add lemon juice, eggs, and vanilla. Mix until thoroughly combined.
Gradually add the flour mixture into the stand mixer bowl and mix until just combined being careful to not over mix.
Pour batter into prepared pan and bake for 15 minutes at 350°F. Reduce oven temperature to 325°F and continue to bake until deep golden brown and skewer in center comes out clean (about 40-45 minutes).
Cool in pan for 10 minutes and then turn onto wire rack.
for the rosmary-infused lemon glaze
Combine sugar, lemon juice and zest to boil in small saucepan making sure to stir occasionally to dissolve sugar.
Reduce heat to low and add rosemary sprig and continue simmer until thickened slightly, about 2 minutes.
Use a toothpick poke to holes on all sides of cake. Drizzle glaze over top of cake and then allow to cool at room temperature at least one hour before serving.