Preheat the oven to 350°F (180°C). Prepare an 8-inch round cake pan by greasing it and lining the bottom with parchment paper.

In a large bowl, combine the plain flour, baking powder, baking soda, salt, and caster sugar. Mix the dry ingredients until they are well blended.

Gently fold in the chopped apples to coat them with the flour mixture.

In another bowl, whisk together the eggs, vanilla extract, vegetable oil, and Greek yogurt until smooth and combined.

Pour the wet ingredients into the dry mixture and gently combine until everything is just mixed. Be careful not to overmix.

Transfer the batter to the prepared cake pan and smooth the top with a spatula.

In a small bowl, mix the topping ingredients: caster sugar, ground cinnamon, and melted butter. Sprinkle this mixture evenly over the batter.

Bake for 30-35 minutes or until a skewer inserted into the center comes out clean.

Once done, let the cake cool in the pan for about 10 minutes. Then, carefully transfer it to a wire rack to cool completely.

For the glaze, whisk the icing sugar with milk until smooth. Drizzle it over the cooled cake for a lovely finish.