Preheat the oven to 170°C (340°F). Grease a loaf tin with butter and lightly dust with flour, tapping out the excess. In a bowl, sift together the flour, baking powder, baking soda, and salt. Set aside.

In a large mixing bowl, beat the softened butter and golden caster sugar together until light and fluffy this takes about 3–4 minutes. Add in the lemon zest (reserving about 1 tablespoon for garnish), and mix until evenly distributed.

Pour in the lightly beaten eggs a bit at a time, mixing well after each addition to prevent curdling. If it starts to split, add a spoonful of the flour mixture to help it come back together.

In a separate cup, mix the buttermilk with the lemon juice. Alternate adding the flour mixture and the buttermilk mixture into the batter start and end with flour. Mix gently with a spatula or on low speed until just combined; do not over mix.

Pour the batter into your prepared loaf tin and level the top. Bake for 45 to 50 minutes, or until a skewer inserted into the center comes out clean. Let the cake cool in the tin for 10 minutes, and then turn it out onto a cooling rack to cool completely.

While the cake is cooling, mix together softened butter, powdered sugar, lemon juice, lemon zest, and milk until smooth and pourable. You can warm the glaze slightly to make it easier to drizzle. Once the cake is fully cooled, spoon the glaze over the top and sprinkle with the reserved lemon zest. Let it set before slicing.