For the Cake:
Preheat oven to 350 degrees. Grease and flour three 6″ round cake pans and set aside.
In a large mixing bowl, whisk together sugar, cake flour, baking powder, baking soda, freeze dried peaches, and salt. Set aside.
In a large measuring cup or mixing bowl, stir together the eggs, egg whites, buttermilk, and vanilla extract. Set aside.
Using a stand mixer with the paddle attachment beat together the butter and granulated sugar until light and fluffy, about 3 minutes.
Add 1 cup dry ingredients and mix on low for 30 seconds.
Then add the buttermilk mixture and mix on low for 30 seconds.
Add another cup of the dry ingredients and mix for 30 seconds.
Add the remaining buttermilk mixture and mix for 30 seconds.
Add the remaining dry ingredients and mix until fully incorporated.
Evenly distribute the batter between the three prepared pans.
Bake for about 25-28 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Cool on a wire rack for 10-15 minutes before removing the cakes from their pans.
For the Filling:
In a large bowl add the lemon juice.
Add the diced peaches and coat the peaches with the lemon juice.
Place the diced peaches in a large saucepan and add the brown sugar.
Bring the mixture to a full boil over medium-high heat, stirring occasionally.
Do this for about 25-30 minutes until the mixture is thick.
Add the 1 cup peach jam and stir until combined.
Set aside to cool.
For the Frosting:
In the bowl of a stand mixer, beat the butter for 2 minutes, until fluffy.
Add in the powdered sugar, vanilla, and salt, and beat until combined, scraping the sides of the bowl as you go, about 1 to 1 ½ minutes.
Spoon in the dulce de leche and beat for an additional 2 minutes, until the buttercream is light and fluffy.
To assemble the cake:
Place one of the cake layers on a cake plate.
Using an offset spatula, spread half the peach jam filling over the cake.
Place the second layer on top of the peach filling.
Add the remaining peach jam filling and spread evenly across the layer.
Top with remaining cake layer.
Frost the cake.
The cake will keep covered at room temperature for 2 to 3 days if your kitchen temperature allows; if it is too hot, the frosting won’t keep well so the cake can be kept, covered, in the refrigerator.