Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans and line the bottoms with parchment paper.

In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set aside.

In a large bowl using an electric mixer, beat the softened butter and granulated sugar on medium-high speed until light and fluffy, about 3-4 minutes. Add the eggs one at a time, beating well after each addition. Mix in the lemon zest, lemon juice, and vanilla extract.

Reduce the mixer speed to low. Add the dry ingredients in three additions, alternating with the buttermilk in two additions, beginning and ending with the dry ingredients. Mix until just combined; do not overmix.

Divide the batter evenly between the prepared pans. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cakes cool in the pans for 15 minutes before inverting them onto a wire rack to cool completely.

While the cakes bake, combine the blueberries, sugar, and 1 tbsp lemon juice in a small saucepan. Cook over medium heat, stirring occasionally, until the berries begin to burst and the mixture simmers, about 5-7 minutes.

Stir in the cornstarch slurry (1 tbsp cornstarch mixed with 1 tbsp cold water). Bring back to a simmer and cook for 1-2 more minutes, stirring constantly, until the compote has thickened. Remove from heat and let it cool completely.

Line the bottom of an 8-inch springform pan with parchment paper. This will be your cheesecake layer mold.

In a large bowl, beat the softened cream cheese with an electric mixer until completely smooth. Add the sifted powdered sugar, vanilla extract, and lemon juice, and beat until smooth and combined.

In a separate, clean bowl, whip the cold heavy cream until stiff peaks form. Gently fold the whipped cream into the cream cheese mixture in two additions until fully incorporated and no streaks remain.

Pour half of the cheesecake mixture into the prepared springform pan. Dollop about 1/4 cup of the cooled blueberry compote over the top. Pour the remaining cheesecake mixture on top and gently spread. Place the pan in the freezer for at least 2-3 hours, or until completely firm. This is crucial for easy assembly.

In a large bowl, beat the softened cream cheese and butter with an electric mixer until smooth and creamy. Gradually add the sifted powdered sugar, one cup at a time, mixing on low speed until combined, then increasing to high speed. Add the lemon zest, lemon juice, vanilla, and salt, and beat until light and fluffy.

If your cake layers have domed tops, use a serrated knife to level them.

Place one lemon cake layer on your serving plate or cake stand. Spread a thin layer of frosting on top.

Carefully remove the frozen cheesecake layer from the springform pan and place it on top of the first cake layer.

Spread the remaining blueberry compote over the cheesecake layer, leaving a small border around the edge.

Place the second lemon cake layer on top. Gently press down.

Apply a thin layer of frosting all over the cake (a 'crumb coat') and refrigerate for 30 minutes to set. This will trap any loose crumbs.

Apply the remaining frosting to the top and sides of the cake, smoothing with an offset spatula. Garnish with fresh blueberries and lemon zest if desired. Refrigerate for at least 1 hour before serving to allow the layers to set.