In a medium saucepan, combine the sugar, eggs, and salt. Whisk until smooth.

Add the lemon juice and blueberries, whisking to combine. Heat over medium-low, stirring frequently until the mixture simmers and thickens.

Strain the blueberry curd through a sieve into a heat-safe non-metal bowl, mashing the blueberries to extract all the juice.

Discard the skins and seeds, then add the cubed butter, stirring until melted.

Cover with parchment paper or plastic wrap placed directly on the surface. Refrigerate until chilled and thickened.

Preheat your oven to 355°F (180°C). Grease three eight-inch baking pans with butter or oil and line the bottoms with parchment paper.

In a large mixing bowl or stand mixer, combine the grated lemon zest, sugar, and salt. Rub the mixture with your fingers until it’s yellow and aromatic.

Add the softened butter and oil, beating until creamy and lightened in color.

Incorporate the eggs one at a time, beating well after each addition.

Add vanilla, lemon juice, and sour cream, mixing until combined.

Sift half of the flour along with all of the baking soda and baking powder into the bowl. Gently fold until no dry streaks remain.

Add the buttermilk, fold gently, then sift in the remaining flour and fold until incorporated.

In a separate bowl, toss the frozen blueberries with 1 tablespoon of flour until coated. Fold these blueberries into the batter, being careful not to overmix.

Divide the batter evenly among the prepared pans. Bake for 20 to 25 minutes, or until a toothpick inserted comes out clean with a few crumbs attached.

Let the layers cool for 10 minutes before flipping them out onto a cooling rack to cool completely.

In a large bowl or stand mixer, beat the softened butter until light and fluffy, scraping down the bowl frequently—about 5-7 minutes.

Add salt and 4 cups of powdered sugar, incorporating half a cup at a time until well mixed. Continue beating until very light and fluffy.

Gradually add the softened cream cheese, mixing until fluffy.

Strain the blueberries from their juice and add the juice into the frosting. Stir in lemon juice to taste.

If needed, adjust thickness with more powdered sugar and mix well. Fold the frosting with a spatula to remove large air bubbles.

Place a small dollop of frosting onto a serving plate and set the first cake layer bottom side down.

Spread a thin layer of frosting on top and pipe a thick border around the edge. Fill the center with ¼ cup of blueberry curd.

Repeat with the next layer.

Place the last layer of cake bottom side up. To stabilize, chill the assembled cake in the freezer for 10 minutes.

Spread frosting around the outside and top of the cake, smoothing it down. Pipe any remaining frosting in a border on top.

Top the cake with additional blueberry curd and decorate with lemon wedges and fresh blueberries.

Slice, serve, and enjoy at room temperature!