Make the Peach Filling: In a medium saucepan, combine chopped fresh peaches, brown sugar, cornstarch, and lemon juice. Stir over medium heat until it boils, then mash gently. Simmer for 5-7 minutes until thickened and reduced by half. Cool in refrigerator.

Prepare Cake Layers: Preheat oven to 350°F (176°C) and grease two 8-inch round cake pans. Combine flour, baking powder, cinnamon, and salt in a bowl. In another bowl, cream butter, brown sugar, and oil until light and fluffy. Add in eggs and vanilla, then gradually mix in dry ingredients and milk until smooth.

Bake the Cake Layers: Divide batter evenly between pans. Bake for 30-35 minutes or until a toothpick comes out clean. Let cool in pans for 10 minutes, then transfer to wire racks to cool completely.

Make the Frosting: Beat softened butter until smooth, then gradually add powdered sugar and mascarpone. Mix until fluffy. Set aside some frosting to decorate and mix in part of the cooled peach filling for flavor.

Assemble the Cake: Cut each cooled layer in half horizontally to create four layers. Layer cake with frosting and peaches in between each layer, stacking carefully.

Frost and Decorate: Apply a thin crumb coat, chill for 20-30 minutes, then frost with remaining frosting. Decorate with sliced peaches and brown sugar crumble, if desired. Store in refrigerator.