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

In a large bowl, whisk together the sifted flour, granulated sugar, baking powder, and salt. Set aside.

In a separate large bowl, using an electric mixer, cream the softened butter until light and fluffy (about 2-3 minutes).

Add the eggs one at a time, beating well after each addition until fully incorporated. Scrape down the sides of the bowl as needed.

In a small bowl, combine the milk, lemon juice, lemon zest, and vanilla extract.

Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the milk mixture in two additions, beginning and ending with the dry ingredients. Mix on low speed until just combined, being careful not to overmix. Overmixing can lead to a tough cake.

Divide the batter evenly among the three prepared cake pans. Smooth the tops with an offset spatula.

Bake for 25-30 minutes, or until a wooden skewer inserted into the center of a cake comes out clean. Do not overbake. Allow the cakes to cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely.

While the cakes are cooling, combine sugar and water in a small saucepan over medium heat. Stir until the sugar is completely dissolved. Remove from heat and stir in the lemon juice. Let cool completely.

In a medium heatproof bowl, whisk together the egg yolks, granulated sugar, lemon juice, and lemon zest.

Place the bowl over a saucepan of simmering water (make sure the bottom of the bowl does not touch the water). Continuously whisk the mixture for 8-10 minutes, or until it thickens enough to coat the back of a spoon.

Remove the bowl from heat and immediately whisk in the cold butter pieces, one at a time, until smooth and fully incorporated. Strain the curd through a fine-mesh sieve into a clean bowl to remove any zest or cooked egg bits. Cover the surface directly with plastic wrap to prevent a skin from forming and refrigerate until firm, at least 2 hours or overnight.

In a small saucepan, combine blueberries, sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the blueberries start to burst and release their juices (about 5-7 minutes).

In a small separate bowl, whisk together cornstarch and water to create a slurry. Pour the slurry into the simmering blueberry mixture, stirring constantly. Cook for another 1-2 minutes until the filling thickens. Remove from heat and let cool completely before using.

In a large bowl, using an electric mixer, beat the cold cream cheese and softened butter together on medium-high speed until completely smooth and no lumps remain (about 3-4 minutes). It's crucial for the cream cheese to be cold to prevent a runny frosting.

Gradually add the sifted powdered sugar, 1 cup at a time, mixing on low speed until combined, then increasing to medium. Once all sugar is added, beat for another 2-3 minutes until light and fluffy.

Add the lemon juice, lemon zest, vanilla extract, and pinch of salt. Beat until just combined. Be careful not to overmix, as this can make the frosting too soft. If the frosting is too soft, refrigerate for 15-20 minutes to firm up slightly.

Once all cake layers are completely cool, use a serrated knife to level the tops of the cakes if necessary, creating flat surfaces.

Place one cake layer on your serving platter or cake stand. Brush generously with the cooled lemon simple syrup.

Pipe or spread a dam of cream cheese frosting around the outer edge of the cake layer. Fill the center with half of the blueberry filling, then dollop half of the lemon curd on top of the blueberries and gently spread.

Carefully place the second cake layer on top, brush with simple syrup, and repeat the frosting dam, blueberry filling, and lemon curd layers.

Top with the third cake layer, brush with simple syrup. Apply a thin layer of cream cheese frosting over the entire cake (top and sides) to create a crumb coat. Refrigerate for 15-20 minutes to set the crumb coat.

Remove the cake from the refrigerator and apply the remaining frosting, smoothing it with an offset spatula. Decorate with fresh blueberries and lemon zest, if desired.

Chill the cake for at least 30 minutes before slicing and serving for best results. Enjoy your Zesty Lemon Blueberry Layer Cake!