Preheat the oven to 170C. Grease two 8-inch cake tins with butter & line the bases with parchment paper. Set aside. Remove 1/3 cup flour & set aside.
Sift together the remaining flour, baking powder, & cinnamon. Set aside.
Using an electric mixer, beat the butter until smooth. Add the sugar, & while the mixer is on medium speed, cream together the sugar and butter until it is light and fluffy. Mix in the vanilla extract.
With the mixer on medium-low, mix in the eggs one at a time, ensuring to scrape down the sides & thoroughly combine after each addition.
Working in alternating batches, mix in half of the dry ingredients followed by the milk. Add in the second half of the dry ingredients & mix until just combined.
Toss the diced apples with the flour you set aside at the beginning, & fold into the batter until evenly distributed.
Divide the batter between the two tins & bake in the preheated oven for 32 to 35 minutes until golden on top & a skewer inserted into the center of the cakes comes out clean. Leave to cool on a wire rack for 15 to 20 minutes before removing the cakes from their tins.
While the cakes are baking, make the browned butter frosting. Use an electric whisk to beat the cooled butter until it is creamy & almost white. Add all of the remaining ingredients & beat until well combined & fluffy. Set aside.
When the cakes are completely cooled (trust me on that one!), slice each in half horizontally, giving you four even layers. Place the first layer on your cake stand & spread an even layer of the frosting across the top. Repeat with the remaining layers. Using an offset spatula, evenly coat the outside of the cake in a lovely layer of frosting. Finish off with some hand picked apple leaves!