Preheat the oven to 375' (350' convection). Line a 9x13" baking dish with parchment paper, or butter and flour to prevent cake from sticking.
In a small bowl, whisk together flour, baking powder, baking soda and salt.
In a stand mixer, beat together butter and sugar with a paddle attachment until light and fluffy, about 2-3 minutes at a moderate speed.
Add eggs one a time, mixing well after each addition. Stir in vanilla and lemon zest.
Turn mixer to low and slowly add the flour and buttermilk, alternating between the two. Just barely mix until all the flour is incorporated. Gently fold the peaches in by hand.
Pour the batter into the prepared pan and smooth with a wooden spoon or spatula. Evenly distribute the raspberries on top of the cake, facing some up and others down so some sink and some float. Sprinkle the top of the cake with the 3 tbs additional sugar.
Bake cake for 20-25 minutes, until lightly golden and a toothpick inserted into the center comes out clean. Cool in pan for 10 minutes before turning onto a cooling rack. Serve warm or room temperature with fresh whipped cream.
Whipped Cream
Whip cream, powdered sugar, vanilla and salt, with a hand mixer or stand mixer fitted with a whisk attachment, 3-4 minutes until stiff peaks form.
Notes
*Buttermilk substitute can be made by putting one tablespoon vinegar or lemon juice (I use apple cider vinegar) in a 1-cup measuring cup, and filling the rest of the way with milk or almond milk. Let sit for a couple of minutes, until the milk looks curdled, then use in recipe as directed.