In a large saucepan, combine 2 cups sugar, cornstarch, and salt, mixing well. Add egg yolks and water to sugar mixture, whisking well to combine. Cook over medium heat, stirring often, until mixture thickens, 15 to 20 minutes. Remove from heat.

Add lemon juice, butter, and 2 teaspoons vanilla, stirring well. Cover and refrigerate for at least 2 hours.

In a large mixing bowl, beat cream and remaining 1/2 cup sugar at high speed with an electric mixer until soft peaks form. Add remaining 1 teaspoon vanilla and beat until stiff peaks form. Set aside.

In the bottom of a trifle bowl, layer half of blueberry muffins. Top muffins with 3 cups blueberries. Top blueberries with half of whipped cream. Top whipped cream with two-thirds of lemon mixture. Top with remaining 3 cups blueberries and remaining blueberry muffins.

In a large bowl, combine remaining lemon mixture and whipped cream, folding gently to combine. Spread over muffin layer.

Garnish with additional blueberries and a lemon-peel curl, if desired.