For the Lemon Cake
Baking spray
1 3/4 cups sugar
2 tablespoons freshly grated zest and 1/4 cup juice from 3 lemons
2 1/4 cups cake flour
2 teaspoons baking powder
1 teaspoon baking soda
3/4 cup milk, at room temperature
1 teaspoon pure vanilla extract
4 large eggs, separated and at room temperature
1 teaspoon salt
2 sticks (8 ounces) unsalted butter, at room temperature

For the White Chocolate Icing
1 1/2 cups sugar
1 tablespoon finely grated zest and 3/4 cup juice from 6 to 8 lemons
1/4 cup all-purpose flour
1/4 cup cornstarch
1/2 teaspoon salt
1 1/2 cups milk
10 ounces white chocolate, finely chopped or 10 ounces white chocolate chips
1 tablespoon pure vanilla extract
2 sticks (8 ounces) unsalted butter, cut into tablespoon-sized pieces and at room temperature

For the Assembly
2 cups blackberry jam or preserves
55 vanilla rolled wafer cookies (See Notes)
10 ounces white chocolate, finely chopped or 10 ounces white chocolate chips
1/2 cup heavy cream
2 tablespoons vegetable shortening
2 teaspoons juice from 1 lemon
1/8 teaspoon salt
6 cups fresh blackberries