The sauce can be made while the cake is cooking or many days in advance.
In a large saucepan over medium heat, combine blueberries, water and sugar. Stir frequently, but careful not to crush the berries, bring to a low boil.
In a small bowl, mix the cornstarch with cold water until combined.
Slowly stir the cornstarch into the blueberries, careful not to crush them. Simmer until the homemade blueberry sauce is thick enough to coat the back of a metal spoon, about 10 minutes.
Remove from heat and gently stir in vanilla.
If making the buttercream frosting, let the sauce cool completely before topping the cake.
Note: this makes a lot of sauce, save the rest for other recipes, or cut it in half.
Mascarpone Lemon Buttercream:
Place softened butter and mascarpone into the bowl of an electric stand mixer that has been fitted with the wire attachment. Turn the mixer on the medium setting and cream until it smooth and combined, 2 - 3 minutes.
Add sugar, ½ a cup at a time. Add lemon zest and a pinch of salt and whisk until well-incorporated.
If the frosting is too thick add heavy cream one tablespoon at a time until it has reached the desired consistency.
Top the cooled cake with the lemon mascarpone buttercream, and pour the cooled blueberry sauce on top of the frosting.