Peel the apples, remove the cores and cut them into thin wedges, approximately 3–5 mm thick. Toss with lemon juice and cinnamon. Add the optional tablespoon of sugar if the apples are tart. Keep the slices reasonably uniform so they cook evenly.
In a large bowl, whisk the eggs and sugar for 2–3 minutes until pale and slightly foamy. Stir in the vanilla extract and melted, cooled butter. Do not add hot butter directly to the eggs.
Sift together the flour, baking powder and salt. Add them to the egg mixture in two additions, alternating with the milk and cream. Whisk gently until smooth. The batter should be thin and pourable, more like a thick custard than a bread or sponge-cake batter.
Preheat the oven to 175°C conventional / 350°F or 160°C fan. Grease a 23 cm pan and line the base with baking paper. Arrange the apple wedges in overlapping layers. Pour the batter evenly over the apples, gently shaking the pan so the cream flows between the slices.
Bake for approximately 45–60 minutes. The apples should be tender and the top golden brown. Insert a thin knife into the center, avoiding a large apple slice. It should come out without raw batter. The center may still have a slight custardy wobble, but it should not be liquid. If the top browns too quickly, cover it loosely with foil for the remaining baking time.
Allow the cake to cool in the pan for at least 30–45 minutes. For cleaner slices and a firmer custard, refrigerate for 2–3 hours after it has cooled. Dust with powdered sugar just before serving. Serve chilled for a firm custard texture or slightly cool for a softer, creamier result.