01 - Preheat oven to 350°F. Line a 9 x 13 inch baking pan with parchment paper.
02 - In a large bowl, beat together the softened butter and caster sugar until pale and fluffy.
03 - Add eggs one at a time, mixing thoroughly after each addition.
04 - Fold in the self-raising flour, poppy seeds, lemon zest, vanilla extract, and salt. Stir in the milk until smooth.
05 - Pour batter into the prepared pan, smoothing the top evenly. Bake for 28 to 32 minutes, or until a skewer inserted into the center comes out clean.
06 - While baking, mix lemon juice with icing sugar until well combined.
07 - Once removed from oven, poke holes throughout the cake with a skewer and pour the lemon drizzle over the hot surface evenly.
08 - Allow the cake to cool completely in the pan before slicing into 12 squares and serving.