Grease a 20cm cake tin/cake pan with butter and line with Parchment paper/Greaseproof paper. Use a loose bottomed or springform pan.
Heat oven to 180 C/ 350 f/Gas Mark 4.
Peel and core the apples and cut into chunks. Place in a medium bowl. Squeeze over Lemon Juice and set aside.
For the Cake, beat Butter, Sugar and Lemon Zest in a large mixing bowl till pale. Or add to the bowl of your electric mixer/stand mixer with a paddle attachment.
Add Eggs 1 by 1 mixing between each one.
Fold the dry ingredients into wet ingredients - Plain Flour, Baking Powder and Salt. Mix on low speed till combined.
Stir in the Buttermilk.
Add the chunks of Apples and fold in gently.
Make the streusel layer by combining the Sugar, Vanilla, Cinnamon, Flour and cold Butter together in a bowl. Use a spatula/food processor or your hands to form breadcrumbs. It should be crumbly. Place in the fridge until needed.
Place the batter into the prepared tin. Add the streusel topping on top of the batter and spread over evenly.
Bake for 40 - 55 minutes. The time depends on apples used and the amount of water they may release.
Check if the cake is done by inserted a skewer into the middle. If it comes out clean, the cake is done.
Let the cake cool for 15 minutes in the tin before removing the warm cake from the tin to a wire rack to cook.