Grind the poppy seeds in a poppy seed mill or food processor until they are crushed, but not ground.
In a small saucepan over medium heat, warm the milk, butter and sugar until almost at boiling point. Add the poppy seeds and cook for 2 minutes, stirring continuously. Remove from the heat, cover and set aside for 15 minutes.
To make the base layer, place all the ingredients in a mixing bowl and knead until a dough comes together. Turn out onto a bench and continue kneading until smooth, about 1 minute. Place in a ziplock bag and refrigerate for 30 minutes.
Preheat the oven to 160°C fan-forced. Line the base and side of a 23 cm round springform tin.
To make the filling, place all the ingredients, except the eggs, in a large mixing bowl with the poppy seed mixture. Beat until smooth using electric beaters. Add the eggs, one at a time, beating well after each addition. Set aside.
To make the crumble, dice the cold butter and place it in a bowl together with the remaining crumble ingredients. Using your fingertips, rub the mixture until it resembles panko breadcrumbs. Refrigerate.
Roll out the rested base dough on a lightly floured bench to a circle 23 cm in diameter. Transfer it to the cake tin and lightly press down. Use a fork to prick about 15 holes in the base. Add the filling and very lightly tap the tin on the bench so the filling settles. Bake in the oven for 40 minutes.
Remove the tin from the oven and add the crumble. Increase the oven temperature to 180°C fan-forced and bake the cake for a further 25 minutes. Remove from the oven and cool in the tin before turning out. Serve with a dollop of whipped cream.
