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.
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.
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.
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.