1. To make the peach filling, add the chopped peaches, brown sugar, cornstarch and lemon juice to a large saucepan. Heat slowly over medium heat until the mixture begins to thicken and boil, then mash to break down the peaches.
2. Continue cooking over medium-low heat and let simmer until the filling is reduced by about 1/2, then remove from heat to cool. The filling will have thickened a bit and will continue to thicken as it cools, though it’s not meant to be a super thick filling. Set filling in the fridge to cool.
3. To make the cake layers, prepare two 8 inch cake pans with parchment paper circles in the bottom and grease the sides. Preheat oven to 350°F (176°C).
4. Combine the flour, baking powder, cinnamon and salt in a medium sized bowl and set aside.
5. Add the butter, brown sugar and vegetable oil to a large mixer bowl and beat together until light in color and fluffy, about 3-4 minutes. Do not skimp on the creaming time.
6. Add the eggs and vanilla extract and mix until completely combined and smooth. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
7. Add half of the dry ingredients to the batter and mix until mostly combined.
8. Add the milk and mix until well combined. The batter will look a little curdled, but that’s ok.
9. Add the remaining dry ingredients and mix until well combined and smooth. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated. Do not over mix the batter.
10. Divide the batter evenly between the cakes pans and bake for 30-35 minutes, or until a toothpick comes out with a few crumbs.
11. Remove the cakes from the oven and allow to cool for about 2-3 minutes, then remove to cooling racks to cool completely.
12. When the filling and cakes are ready and cooled, make the frosting. Place the butter in a large mixer bowl and beat until smooth.
13. Add about half of the powdered sugar and mix until well combined and smooth.
14. Add the mascarpone cheese and mix until well combined and smooth.
15. Add the salt and peach filling and mix until well combined and smooth.
16. Add the remainder of the powdered sugar and mix until well combined and smooth.
17. To put the cake together, cut both cakes in half lengthwise so that you end up with 4 layers of cake total. To do this, I use a long serrated knife.
18. Place the first layer of cake on a cake plate or cardboard cake circle and top with about 1/3 cup of the frosting. Spread the frosting into an even layer.
19. Pipe a dam of frosting around the edge of the cake and then fill it with roughly 1/3 of the remaining filling.
20. Add the second layer of cake, then repeat another layer of frosting and filling.
21. Add the third layer of cake, then another layer of frosting and filling.
22. Add the final layer of cake on top and then add a thin crumb coat of frosting to the outside of the cake. Refrigerate the cake until firm and sturdy, about 1-2 hours. Set the remainder of the frosting in the fridge and take it out about 30 minutes before you’re ready to continue, so it can soften a bit.
23. Frost the outside of the cake with the remainder of the frosting, then finish it off with some sliced peaches and brown sugar crumble, if you like. To make the brown sugar crumble, combine the flour, brown sugar and butter in a medium-sized bowl and use a fork to press the butter into the dry ingredients until it all forms a crumbly mixture. Sprinkle the mixture over a lined cookie sheet and bake at 350°F (176°C) for about 5-8 minutes, until the crumble pieces starts to brown around the edges. Remove and set aside to cool, then add to the cake.
24. Cover and refrigerate the cake until ready to serve. I recommend setting the cake out for about an hour or so before serving so it’s not quite so cold.