Preheat the oven to 170°C. Prepare a pan measuring 21 cm x 28 cm (or another with the same surface area, e.g. 24 x 24 cm), grease it with butter and line it with parchment paper.
Place the butter in a saucepan and melt over very low heat. Add the chocolate broken into small pieces and melt, stirring constantly. Remove from the heat, being careful not to overheat the mixture.
In a separate bowl, beat or whisk the eggs with the sugar. Add the melted chocolate and butter along with the salt, and mix or whisk until smooth.
Add the flour mixed with the baking powder and mix into a uniform batter. Pour into the prepared pan, arrange the pieces - slices of peeled apple - on top, and dust them with cinnamon and vanilla or cane sugar. Scatter the extra chocolate pieces (50 g) over the top.
Bake for about 40 minutes, until the cake rises and a crust forms on top. If using a larger pan, it will be ready sooner; if using a smaller one, extend the baking time.
Once baked and cooled, cut into small pieces. Serve with caramel sauce or "salted caramel" sauce (homemade or store-bought).
