01 - Preheat the oven to 350°F. Line a 9 x 13 inch baking tray with parchment paper.
02 - In a large bowl, cream the butter and caster sugar until pale and fluffy.
03 - Beat in the eggs one at a time, ensuring each is fully combined before adding the next.
04 - Fold in self-raising flour, poppy seeds, lemon zest, vanilla extract, and salt. Stir in the milk to achieve a smooth batter.
05 - Pour batter into the prepared tray and level the surface. Bake for 28 to 32 minutes, or until a skewer inserted in the center comes out clean and the top is golden.
06 - While baking, combine lemon juice and icing sugar to form the drizzle.
07 - Immediately after removing the tray from the oven, pierce the surface with a skewer and pour the lemon drizzle evenly over the warm cake.
08 - Allow the traybake to cool completely in the tin before cutting into 12 squares to serve.