Preheat oven to 180ºC (356ºC). Line a 9” x 5“ rectangle baking pan with parchment paper, set aside. NOTE: Alternatively, if you have a non-stick pan, liberally grease it with melted butter and dust it with a light layer of flour. This will leave your finished loaf with smooth edges.
In a large mixing bowl, or the bowl of an electric stand mixer (if using), add softened butter and caster sugar. Beat on high for 1-2 minutes or until mixture turns light/pale yellow and fluffy in texture. Scraping down the sides as needed.
Add eggs one at a time, beating after each addition until just combined.
Beat through vanilla and lemon zest until just combined.
Add flour and baking powder. Beat until just combined.
Add lemon juice and beat until well combined and you have a thick smooth batter.
Pour into lined pan and smooth out top.
Bake for 40-50 minutes or until a toothpick inserted into the middle comes out clean. The top should be slightly cracked and golden.
Remove and cool in pan for 5 minutes.
Carefully remove and turn out cake onto a wire rack to cool completely.
Slice, serve and enjoy!