Begin by preheating your oven to 350°F (175°C) and preparing a greased baking pan. In a large mixing bowl, cream together 125 g of softened butter with 1 cup of caster sugar until light and fluffy, which typically takes about 2-3 minutes of beating.
Add 2 eggs one at a time, ensuring each is fully incorporated before adding the next, then mix in 2 teaspoons of vanilla essence. In a separate bowl, combine 2 cups of gluten-free self-rising flour with 1 teaspoon of gluten-free baking powder and 2 teaspoons of ground cinnamon, whisking together to distribute the leavening agents and spices evenly.
Gently fold the dry ingredients into the wet mixture, alternating with 1/2 cup of milk, beginning and ending with the dry ingredients to prevent overmixing.
Pour the batter into your prepared pan and arrange 3 cups of diced fresh apples or tinned apples over the top, pressing them gently into the batter. For the topping, melt 1/2 cup of butter and combine it with 1/4 cup of raw sugar and 1/4 cup of ground cinnamon, stirring until well combined.
Drizzle this butter-cinnamon-sugar mixture evenly over the apples and batter. Bake for 35-45 minutes, until a skewer inserted into the cake portion comes out clean and the apples are tender. The top should turn golden brown and slightly caramelized from the cinnamon-sugar topping.
Allow the cake to cool in the pan for 10-15 minutes before turning out onto a wire rack to cool completely.