Combine blueberries, sugar, and lemon juice in a medium saucepan over medium heat. Cook for 8-10 minutes, mashing berries occasionally, until they break down and the mixture bubbles vigorously. Stir in the cornstarch slurry and cook 1 minute more until thickened. Remove from heat and let cool completely, about 30 minutes. You should have about 1 cup of jam.

Preheat oven to 350F. Butter and flour two 9-inch round cake pans, tapping out excess. Whisk together flour, baking powder, and salt in a medium bowl.

Beat butter and sugar in a large bowl with an electric mixer on medium-high speed until pale and fluffy, about 4 minutes. The mixture should look almost white and feel light when you scrape the bowl.

Beat in eggs one at a time, mixing well after each addition. Add vanilla and beat 30 seconds. The batter should look smooth and satiny, not curdled.

Reduce mixer to low. Add flour mixture in three additions, alternating with milk in two additions, beginning and ending with flour. Mix just until combined after each addition. Do not overmix - a few streaks of flour are fine.

Divide batter evenly between prepared pans and smooth tops. Bake 28-32 minutes, until cakes spring back when lightly touched and a toothpick comes out clean. Cool in pans 10 minutes, then turn out onto wire racks to cool completely.

Beat heavy cream, powdered sugar, and vanilla in a chilled bowl on medium-high speed until soft peaks form, about 3 minutes. The cream should hold a gentle curl when you lift the beaters - stop before it gets stiff.

Place one cake layer on a serving plate. Spread with half the cooled blueberry jam, then half the whipped cream. Top with second cake layer. Spread remaining jam on top, leaving a 1-inch border, then dollop remaining cream in the center. Swirl gently with the back of a spoon. Serve immediately or refrigerate up to 4 hours.