01 - Preheat the oven to 350°F. Line a 9 x 13 inch baking pan with parchment paper.
02 - In a large bowl, beat the butter and caster sugar until pale and fluffy using an electric mixer or wooden spoon.
03 - Beat in the 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 the batter is smooth.
05 - Pour the batter into the prepared pan and level the surface evenly.
06 - Bake for 28 to 32 minutes or until a skewer inserted into the center comes out clean.
07 - While baking, combine lemon juice and icing sugar to create the drizzle.
08 - Immediately after removing from the oven, poke holes all over the hot cake with a skewer and pour the drizzle evenly over it.
09 - Allow the traybake to cool completely in the pan, then cut into 12 squares to serve.