Preheat oven to 350°F (175°C). Grease a 9-inch tart pan with a removable bottom with butter or grease, and line the bottom of the pan with parchment paper, cut to size. This will make removing the tart very easy. Place the tart pan on a small baking sheet so that it’s easier to move in and out of the oven.

Pulse graham crackers and brown sugar in a food processor until finely ground. You should have about 1½ cups of crumbs. Pour in melted butter while running the processor at low speed until the mixture resembles wet sand. Press the crumbs firmly and evenly into the bottom and up the sides of the tart pan. Bake the crust for 10-12 minutes or until lightly golden and fragrant. Keep the oven on.

In a medium saucepan, combine blueberries and lemon juice. Cook over medium heat, stirring occasionally, until blueberries burst and release their juices, about 8-10 minutes. Strain mixture through a fine-mesh sieve into a bowl, pressing firmly to extract as much liquid as possible. Wipe out the saucepan.

Whisk together the sugar and eggs in a small bowl until they are lighter in color. Return the strained blueberry juice back to the clean saucepan and place over medium heat. Once it starts to steam and simmer, scoop out ½ of the juice, and pour it into the beaten eggs while whisking quickly. This will temper the eggs so they won’t cook and curdle when you add them to the blueberry juice.

Gradually whisk the tempered eggs into the saucepan with the rest of the blueberry juice, and continue whisking until the mixture has a lighter color and has thickened, about 7 minutes. Remove from heat and stir in room temperature butter, one tablespoon at a time, until all of the butter has melted and is mixed in thoroughly.

Pour the curd into the baked crust, and bake the tart for 10 minutes at 350°F to help the filling set faster. Press plastic wrap directly onto the surface of the curd to prevent a skin from forming, and chill the tart in the refrigerator for at least 4 hours or until the filling is completely set.

Garnish with fresh blueberries and powdered sugar, if desired, and serve chilled.