Preheat oven to 325F. Butter and line a 9x5" pan with parchment paper with overhang sides for easy removal.
In a large mixing bowl, cream the butter and 1 cup of sugar together for 5-7 minutes until light and fluffy.
1 cup (227g) unsalted butter , 1 cup (200g) granulated sugar
Beat in the lemon zest and eggs, one at a time, mixing well in between.
zest of 1 lemon, 3 large eggs
In another bowl, toss together the flour and baking powder. Mix in half of the flour mixture, followed by the lemon juice, then the remaining flour mixture until just combined.
2 & 1/2 cups (315g) all-purpose flour, 2 tsp baking powder, juice of 1 lemon
Transfer the batter into the prepared pan. Use a butter knife and cut a line lengthwise down the center of your batter. Sprinkle the remaining sugar on top.
2 Tbsp (25g) granulated sugar
Bake for 50-60 minutes or until a toothpick inserted into the center comes out clean. Let cool for 10 minutes in the pan before removing to a wire rack to cool completely.
Notes
Leftovers can be tightly wrapped or placed in an airtight container and stored at room temperature for 3-4 days.