Set your oven to 180°C (350°F) and grease and line a 9x13-inch traybake tin (or similar size) with parchment paper.

In a large mixing bowl, beat together the softened butter and sugar until pale and fluffy. You can use an electric mixer for this step.

Beat in the eggs, one at a time, along with the vanilla extract. If the mixture starts to curdle, add a tablespoon of the flour to bring it back together.

Sift the flour, baking powder, and cinnamon (if using) into the bowl. Gently fold everything together, adding the milk gradually until you have a smooth, soft batter.

Peel, core, and thinly slice the apples. Mary Berry often recommends tossing them in a little lemon juice to prevent browning.

Spoon half the batter into your prepared tin and spread it out evenly. Arrange half the sliced apples on top. Then, add the remaining batter and spread it gently. Finally, layer the rest of the apples on top, pressing them slightly into the surface.

Sprinkle demerara sugar over the top — this will give a lovely crunch and caramelized finish once baked.

Bake for 40–45 minutes, or until the cake is golden brown and springs back when lightly touched. A skewer inserted into the center should come out clean.

Cool and serve

Let the traybake cool in the tin for about 10 minutes before transferring to a wire rack. Cut into squares or slices, and serve warm or at room temperature.