Preheat the oven to 180C/350F and spray an 8x8 inch square pan with cooking spray. Place one strip of grease proof paper across the pan, spray with cooking spray and then place another strip perpendicular to the first one.
Place your sliced apples into a shallow bowl and cover with water. Add in the lemon juice, mix briefly with your hands and leave to one side.
Place the butter and sugar into a large bowl, or the bowl of your stand mixer and beat until light and fluffy, around 2 minutes on medium speed.
Add in the eggs and vanilla extract, scraping down the sides as necessary, and mix on medium speed until well combined, around 1 minute.
Add in the salt, baking powder, cinnamon and flour and mix on low until the mixture begins to come together. Then add in the clotted cream and continue to mix on low-medium speed until everything is smooth and well combined.
Tip the batter into your prepared pan and smooth into an even layer with a spatula/back of a spoon.
Drain the apples in a sieve and then place them into a bowl lined with kitchen paper. Dry off the slices of apple and then place slices into the top of the cake. I did this in three strips, I left a gap around the edge of the cake, then made one strip of apples before turning the apple slices 180 to go back the other way. I finished with a third strip of apples going the same way as the first strip. You don't have to do this, but I thought it looked pretty!
Place in the oven at 180C/350F for 25 minutes, then turn the oven down to 160C/325F and cook for 15-20 minutes, until a skewer comes out of the centre clean and the middle doesn't look wet anymore. Even though you dried the apples, they still add a little moisture to the mixture!
Leave to cool completely, before using the strips of grease proof paper to lift the cake out. Trim the edges and then slice into 9 bars. I cut along the apple strips and then cut each strip into three.
Apple and Cinnamon Clotted Cream Cake will keep in an airtight container, in the fridge for 3 days, but is best served at room temperature.