Preheat the oven to 350 degrees Fahrenheit and grease 2 (8-inch) cake pans and line the bottoms with parchment paper, set aside.

Cream together the butter and sugars in the bowl of a stand mixer fitted with the paddle attachment at medium speed until smooth and fluffy. Add the eggs, one at a time, beating well after each addition. Lower the speed and add the buttermilk and vanilla extract, mixing until combined. Add the flour mixture and mix until a batter forms, then stir in the strawberries, peaches, and thyme until just incorporated.

Evenly distribute the batter between the 2 (8-inch) cake pans and bake until lightly golden and a toothpick inserted into the center of the cake coms out relatively clean (unless you poked through  fruit bit), about 35 to 40 minutes. Remove and allow to cool for 15 minutes before running a spatula around the edge of the pan and flipping the pan over onto a wire rack, removing the pan and allowing the cakes to cool completely on the racks.

For the mascarpone whipped cream, beat together the mascarpone, whipping cream, sugar, and vanilla extract in the bowl of a stand mixer fitted with a whisk attachment at medium high speed until fluffy, about 1 to 2 minutes.

Cut each of the cake layers in half horizontally using a sharp bread knife, you should now have 4 layers. Divide the mascarpone whipped cream into 4 parts and spread out 3 parts between the layers of cake along with the fresh strawberry and peach slices. Place the remaining fourth of the mascarpone whipped cream on top of the cake in a dollop and arrange fresh fruit or flowers around it, serve immediately. (Note, if you are using inedible flowers as a decoration, leave them at that and make sure everyone knows not to eat them).