In a saucepan, combine the brown sugar, butter, heavy cream, and vanilla extract. Cook over medium heat for about 3-5 minutes, until the mixture darkens and the bubbles become larger and less rapid. Don’t overcook or it will become more like chewy caramels.

Remove from the heat and stir in the salt. Allow the mixture to cool until warm but still pourable before pouring it over the cake. If it cools down too much and is no longer pourable, reheat it in the microwave for 10 seconds and stir.

Preheat the oven to 175°C (350°F). Line the bottom and sides of an 8-inch (20 cm) springform pan with parchment paper.

In a bowl, combine the flour, baking powder, cinnamon, and salt. Set aside.

In another bowl, whisk together the eggs, brown sugar, and vanilla extract until well combined and emulsified.

Add the melted butter, vegetable oil, and milk, then mix to combine.

Add the dry ingredients and mix just until no dry flour remains and the batter is smooth.

Pour the batter into the prepared pan and spread the cubed apples evenly on top, pressing gently so the apples stick to the batter.

Bake for 45–50 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Remove the cake from the oven and let it rest for 10 minutes.

To remove the cake without breaking it, remove the sides of the pan and place a plate on top. Flip it upside down, remove the bottom of the springform pan, and peel off the parchment paper. Then place a serving plate on the bottom of the cake and flip it again so it is right side up.

Pour or brush the warm toffee sauce evenly over the cake. Serve while still warm.