Preheat the oven to 350°F/180°C. Line a 9x3 round cake pan or springform pan with parchment paper. Butter or spray both the paper, and the sides of the pan. I believe butter gives a better aroma whilst baking instead of an oily chemical aroma.
Peel and core the apples, chop in quarters, then slice into thin bite size pieces.
Place the apples into the buttered cake pan.
Using a mixer or whisk, beat the eggs and sugar in a bowl until the sugar dissolves (around 5 minutes).
Add the vanilla and mix throughout mixture.
Gently add the flour and mix with a spoon until batter is thick and smooth.
Pour over the apples in the cake pan, spreading it evenly over the apples. Gently shake the pan to allow the batter to go down to apples.
Bake in the oven for 40-50 minutes. Check cooking time at 40 minutes with a toothpick to see if cake is ready (Some ovens this is enough cooking time). Tip: Don't rush the cooking time, or the centre will remain raw.
When cake is ready, allow to cool on a baking tray for 10 minutes.
Using a knife, run along sides of the cake pan. Carefully flip upside down onto a baking tray. Peel off the parchment paper, revealing the apple base (now the top of your cake).
Sprinkle generously with cinnamon sugar to taste, and transfer to a cake stand or plate. Serve with thickened cream or vanilla bean ice cream.