Preheat your oven to 180 degrees Celsius. Take an 8 inch baking pan and grease it with butter and dust with flour.
To make the streusel topping:
Take a bowl, add the flour, sugar, ground cinnamon, and salt to mix together. Cut the butter and add it into this mixture to resemble coarse crumbs. Then put in the walnuts and stir well together. Keep this aside.
To make the Cake Batter:
In a separate bowl, add the flour, baking powder, salt and whisk it well together.
Also beat the butter separately in the electric mixer or with the help of a hand mixer, until it is smooth and creamy. Then add the sugar and beat till its light and fluffy. Next, add the egg and vanilla extract and beat it well together.
Now, pour in the the flour batter into this butter mixture by alternately pouring in the milk and beat till it is combined well.
Carefully pour this batter into the prepared pan and press the top part gently with the back of a spoon.
Next, arrange the apple slices beautifully on top of this batter and sprinkle the streusel topping generously on top.
Bake this cake for 45 - 50 minutes or till a toothpick inserted in the center comes out clean. Once cooked, remove it from the oven and place on a wire rack to let it cool.
Serve warm with a dash of icing sugar sprinkled on top.