Preheat the oven to 350 degrees F. Grease and flour an 8 inch round cake or springform pan. If using a cake pan, line the bottom with parchment paper before flouring the pan.
In a medium bowl, whisk together the flour, baking powder, salt and cinnamon.
Using a hand mixer or stand mixer, cream together the butter and brown sugar until pale and fluffy, about 3-4 minutes.
Beat in the eggs and vanilla extract until well combined. Don’t worry if it looks curdled at this point.
Add the flour mixture and mix on low speed until just combined.
Stir in the apples, until evenly distributed.
Bake in the preheated oven for 40-45 minutes, until a toothpick inserted in the centre comes out clean. The cake will be quite dark, because of the brown sugar, but don't worry it's moist and delicious.
Allow the cake to cool in the pan for about 20 minutes. If using a cake pan, run a knife between the pan sides and cake, turn out onto a wire rack and remove the parchment paper. If using a springform pan remove the sides.