Peel and core the apples, then cut them into eighths and put them into a saucepan with castor sugar and verjuice. Bring to a boil over a high heat, and gently stir until the sugar has dissolved. Turn off the heat and leave the apples in this light syrup for 10 minutes, then drain, reserving the syrup.
Preheat the oven to 180°C and grease and line a 22 cm spring form tin. To make the cake, cream the butter and castor sugar until pale and thick. Add the eggs one at a time, beating well after each addition. Sift the flour, bicarbonate of soda, cream of tartar and salt and add alternately to the egg mixture with the milk and vanilla extract. Spoon the batter into the prepared tin. Arrange the apple on the top in a concentric pattern, pushing the pieces into the batter slightly. Mix 2 teaspoons of castor sugar and the ground cassia and sprinkle evenly over the top. Bake for 45 minutes or until a skewer inserted into the centre of the cake comes out clean. Let the cake stand for at least 10 minutes before removing the tin.
While the cake is cooking, make the syrup. Have iced water ready in the sink or a large bowl. Bring the verjuice and castor sugar to a boil in a saucepan and cook until a deep, golden brown. Remove the caramel from the heat and cautiously add the reserved apple cooking syrup – the mixture may spit a little. Stand the saucepan in the iced water immediately to arrest the cooking, then pour into a jug ready to serve.
To serve, brush the warm cake with the syrup. Put a wedge of warm cake onto each plate (one with a gentle lip is best), then ladle over the syrup and offer with a hearty dollop of thick cream.