To make the cake, butter two 9-inch (23 cm) cake pans, line the bottoms with parchment paper and butter the parchment too. Sprinkle the sides with flour. Preheat the oven to 350°F (175°C).
In a medium bowl, whisk together flour, salt, baking powder and soda. Set aside. In a small bowl whisk together the milk and sour cream and set that aside too.
In a large bowl (or in your mixer) beat butter until creamy and fluffy. Gradually add sugar and beat until incorporated. Add eggs one at a time, beating well after each addition. Lastly whisk in the vanilla paste.
With the mixer set on low, or by hand, alternately add the flour and milk mixtures, starting and ending with flour (so you add flour - milk - F - M - F).
Divide the batter and pour it into the prepared pans. Smooth the surface with a spatula and bake each cake for 25-30 minutes or until the cake is golden brown and a toothpick inserted in the center of the cake comes out clean.
Cool cake for 10 minutes, then remove it from the pan (run a knife around the edges, invert cake onto a plate, remove parchment paper and then invert again on a large plate). Cool completely. At this point, you can wrap the cake layers in plastic wrap and store them at room temperature for 1 day or in the freezer for up to 3 months.
Make the blueberry sauce
As the cakes bake, you can make the sauce. Combine all the ingredients in a saucepan set over medium-high heat. Bring the mixture to a boil, stirring occasionally. Cook for a few minutes. The liquid should become purple just as the blueberries, thick and the blueberries soft.
Remove from heat, set aside and cool completely.
Prepare the filling
Using an electric mixer and a large bowl, beat the cream with sugar on low-medium speed until soft peaks form. Add mascarpone, along with lemon zest and vanilla.
Mixing on low speed, whip cream and mascarpone until the mixture is smooth. (Do not overbeat or your mascarpone will curdle.) Fold in the blueberries.
Assemble and frost the cake
First level each cake (if needed) and split each cake layer in two, so you get 4 equal layers.
Put the first layer on a large plate and brush some blueberry sauce over it. Spread 1/3 of the filling over the cake. Repeat with two more layers. Cover with the last, 4th, layer. Transfer the cake to the refrigerator and let the filling set for 1 hour, while you make the frosting. (TIP: Use a 9-inch cake baking ring when filling the cake to prevent the filling from coming out the sides. Remove the ring before your frost the cake.)
Make the frosting as the cake chills: Melt white chocolate over a double broiler. Remove it from heat and allow to cool in the bowl for about 15 minutes.
Whip the cream until soft peaks form. Fold in the melted chocolate and thawed wild blueberries, gently whisk until incorporated. Spread the frosting over the cake, starting at the top and then brushing it down the sides.
Chill cake in the refrigerator until frosting sets, then decorate it with fresh blueberries and chill until you are ready to serve. Cake keeps, covered in the refrigerator, for up to 3 days.
Notes
Cake layers and blueberry sauce can be made ahead of time and kept in the refrigerator. Chilled and rested cake layers also have a more manageable crumb and are easier to frost.