Preheat oven to 350º F. Coat 2 9-inch cake pans with cooking spray and line the bottom of the pans with parchment paper cut into rounds.
Place the butter and sugar in the bowl of a mixer fitted with a paddle attachment and beat for about 2-3 minutes or until light and fluffy.
Add eggs, one at a time, beating after each addition.
Add egg yolks, one at a time. Stir in the vanilla.
Stir together salt, baking powder, and flour in a large bowl. Pour half of the flour mixture into the butter mixture and beat until just combined. Add half of buttermilk, then add the remainder of the flour mixture followed by the remainder of the buttermilk. Do not over mix.
Divide the cake batter evenly between the pans.
Bake for 30-35 minutes until a toothpick inserted into the center comes out clean.
Remove from the oven, place the pans on a wire rack and allow the cakes to cool for 10 minutes. Then, turn the cakes out on the cooling rack to cool completely.
For the berry syrup:
Combine the raspberry jam and water; heat in the microwave for 20 second increments or until melted. Press through a sieve to remove seeds.
For the frosting:
With a mixer fitted with the whisk attachment, beat heavy cream and vanilla extract until stiff peaks form. Transfer the whipped cream to a bowl.
Put the mascarpone cheese, cream cheese, butter and confectioners’ sugar in the mixing bowl and beat until very smooth.
Gently fold whipped cream into mascarpone mixture until completely incorporated. Use immediately.
To assemble the cake:
Split each layer in half so that you have 4 layers. Place the first layer on your serving plate.
Brush the berry syrup over the cake layer then add a layer of frosting (approximately 1/2 cup).
Arrange 2/3 cup of berries over the frosting layer; if you're using larger berries such as strawberries, cut them in half first.
Place the second cake layer on top. Brush the second layer with syrup and repeat the process from the first layer of adding the frosting and berries.
Add the third layer of cake, brush with syrup, spread with frosting and arrange the remaining berries on top.
Place the final layer of cake on top. Use the remaining frosting to cover the cake, smoothing the frosting with an offset spatula.
Arrange additional berries and mint sprigs on top. Cake can be refrigerated for one day. Bring to room temperature before serving.