Preheat the oven to 350 F. Line and grease three 6 inch cake pans or two 9 inch cake pans with parchment paper and olive oil.
Melt the butter in a small saucepan. Add the butter, olive oil, thyme, and sugar to a large mixing bowl and whisk until combined. Add the eggs and vanilla extract and whisk again. Add the baking powder, baking soda, and salt and whisk until completely combined, making sure no lumps or bits of the powders remain.
Add 1 cup of the flour and stir with a rubber spatula, carefully folding and combining the flour until most of the streaks of flour are dissolved, but some streaks still remain. Add ½ cup of buttermilk and stir again until both the flour and buttermilk are combined. Repeat with the remaining flour and buttermilk, stirring gently after each addition.
Transfer the batter to your greased and lined cake pans. Bake for 30-40 minutes, or until the edges have released from the sides of the pan and the cake center is bouncy to the touch. Let the cakes cool in the cake pan for 10 minutes before using an offset spatula to release the sides of the pan. Invert the cakes onto a plate or wire rack to further cool.
Make the cream while the cakes cool: Place the cream cheese, mascarpone cream, powdered sugar, salt, and vanilla in a large bowl or stand mixer. Whip everything using an electric mixer until the creams are homogenous and thick, about 5 minutes.
Scrape down the sides of the bowl using a rubber spatula. Add the heavy cream to the bowl and set the mixer on low to incorporate the heavy cream into the mix for 2 minutes. Once homogenous, set the mixer speed to medium, whipping the cream into stiff peaks, about 1 minute. Set cream in the fridge until ready to assemble.
Assemble the cake, layering the cake with cream, roasted grapes, and more cake. Garnish with fresh grapes and thyme, then serve!