1.To make poached quince, stir water, sugar, cinnamon stick and juice in medium saucepan over low heat until sugar dissolves. Add quince and bring to boil. Reduce heat; simmer, covered, about 1 hour 30 minutes or until quince is tender and rosy in colour. Cool quince in syrup to room temperature and strain quince over a medium bowl. Reserve quince and syrup separately.
2.Preheat oven to 180°C (160°C fan-forced). Grease a deep 23cm square cake pan and line base and sides with baking paper.
3.In a small bowl, beat butter and sugar with electric mixer until light and fluffy. Add eggs, one at a time, beating between additions until just combined. Transfer mixture to a large bowl. In two batches, stir in sifted flour and milk. Spread mixture into pan and bake 25 minutes.
4.Meanwhile, blend or process ingredients for cinnamon crumble, pulsing until ingredients just come together.
5.Remove cake from oven. Working quickly, toss frozen blackberries in cornflour to coat. Top cake with drained quince then blackberries, then sprinkle cinnamon crumble over fruit. Return to oven and bake 20 minutes.
6.Stand cake 5 minutes. Turn, top-side up, onto wire rack. Serve cake warm or cold with reserved quince syrup.