Preheat oven to 180°C and line a 22cm (9 inch) square cake tin with baking paper.
Combine poppy seeds and milk in jug and allow to stand, soaking for 10 minutes.
Using the stand mixer fitted with the paddle attachment, beat the butter, rind and sugar on medium speed until light and fluffy. Then continuing on medium speed add eggs one at a time until combined.
Add flours, sour cream and poppy seed mixture in two batches and continue to beat on low speed until combined. Then spread the mixture into the cake tin and bake for 50 minutes or until a skewer inserted into the cake comes out clean. Meanwhile make the lime syrup.
Stir the ingredients into a small saucepan over medium heat. Without boiling continue to stir the syrup until the sugar has dissolved. Then simmer uncovered for 5 minutes.
Allow the cake to stand for 5 minutes then transfer to a wire rack with a tray underneath. Using a skewer, place small holes in the top of the cake before pouring hot lime syrup over the hot cake.