For the Cake: 

1. Preheat the oven to 350 degrees. Grease three 9-inch cake pans (I use a baking spray), line the pans with parchment paper and then spray the parchment paper.
2. Whisk together the flour, salt, baking powder, and baking soda and set aside.
3. Using a handheld or stand mixer fitted with the whisk or paddle attachment beat together the sugar and butter on high speed until creamy and smooth, about 3 minutes. Scrape down the sides of the bowl with a rubber spatula, if needed.
4. Mix in the eggs, egg whites, and vanilla extract on high speed until combined, about 2 minutes. 
5. With the mixer on low speed, add the dry ingredients until just combined. With the mixer still running on low, pour in the buttermilk and continue to mix until just combined.
6. Pour the batter evenly into cake pans. Bake until the cakes are baked through and starting to pull away from the sides of the pan, about 25 minutes. To test if the cake is cooked, insert a skewer or a toothpick into the center of the cake. If it comes out clean, it’s done!
7. Allow the cake layers to cool completely in their pans set on a wire rack. Remember, the cakes must be completely cool before frosting and assembling. If they aren’t, the frosting will melt!

For the Filling: 

1. Add the peaches, brown sugar, and cardamom to a medium saucepan over medium-high heat. Bring the mixture to a boil and then reduce the heat to a simmer and continue cooking, stirring occasionally, for about 10 minutes.
2. Using a potato masher or a fork, smash the peaches until the mixture is mostly pureed and only small chunks of fruit remain. Continue to cook until the jam has reduced and slightly thickened, about 5-7 minutes.
3. Remove the mixture from the heat and let it cool. It should thicken as it cools.

For the Frosting:

1. Using an electric mixer or a stand mixer fitted with a paddle attachment beat the butter, sugar, vanilla, and salt until smooth. Scrape down the bowl, if needed.
2. Add the mascarpone cheese and mix until combined. If the frosting seems too thick, add 1 tablespoon of milk (half & half or heavy cream) and mix until smooth. 

To Assemble: 

1.  Place one cake layer onto your cake plate (or cake turntable, if you’re using one). Using an offset spatula, evenly cover the top with about 1/4 of the frosting. Top that with about 1/4 cup of the peach filling.
2. Top that with a 2nd cake layer. You might want to invert this layer so the top of the cake is flat and smooth. Cover this layer 1/4 of the frosting and 1/4 cup of the peach filling. Repeat with the third layer.
3. Spread the remaining frosting all over the top and sides, using a bench scraper to get that smooth finish.
4. You might want to add a crumb coat (a very thin layer of frosting) to the cake before adding your final layer. This will seal in all those crumbs. If you do want to do this, just add the thin layer and then put the cake in your fridge to chill for about 10-15 minutes before adding your final layer of frosting. 
5. Refrigerate that cake for about 30 minutes before slicing. Then just add your decorations and serve.
6. You can store any leftover cake in an airtight container in the refrigerator for up to 5 days.