Preheat oven to 350F and grease and flour three 6" cake rounds, line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined.
Using a stand mixer fitted with the paddle attachment, beat butter until smooth.
Add sugar and beat on med-high until pale and fluffy (2-3mins).
Reduce speed and add eggs one at a time, fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition. Do not overmix.
Spread batter evenly into prepared pans. Smooth the top with a spatula.
Bake for 30-35 mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack to cool completely.
Blueberry Sauce:
Combine blueberries, water, sugar and lemon juice in a medium saucepan. Bring to a low boil over medium heat. Cook for 2 minutes, stirring occasionally being careful not to break the blueberries.
Slowly add cornstarch mixture while stirring blueberries. Bring back to a boil and cook until thickened (30 - 60 seconds). Cool completely before using. If too thick, add some water to thin out.
Whipped Cream:
Place mixer bowl and whisk into the fridge to chill. Pour cold whipping cream and sugar into the mixer bowl. Whip on high until stiff stiff.
Assembly:
Place one layer of cake onto a cake stand or serving dish. Top with 1/3rd of the whipped cream then 1/3rd of the blueberry sauce. Sprinkle with 1/2 cup fresh blueberries. Repeat for remaining layers. Serve immediately or place in the fridge. Serve same day.