In a medium bowl, beat the softened cream cheese, 3 tablespoons sugar, lemon zest, and 1 teaspoon vanilla with a hand mixer until smooth and fluffy, about 1-2 minutes. Gently fold in 3/4 cup blueberries, reserving the remaining 1/4 cup for topping.

Using a sharp serrated knife, cut a pocket into each bread slice by inserting the knife horizontally into one side, creating a pocket that goes almost to the edges but leaves a 1/2-inch border on three sides.

Carefully stuff each pocket with 2-3 tablespoons of the cream cheese mixture, using a spoon to push it deep into the pocket. Gently press the opening closed with your fingers.

In a shallow dish or pie plate, whisk together the eggs, milk, 2 tablespoons sugar, cinnamon, and 1/2 teaspoon vanilla until well combined and the sugar dissolves.

Heat a large skillet or griddle over medium-low heat and add 1 tablespoon butter. While the butter melts, dip 2-3 stuffed slices into the custard mixture, letting each side soak for 45-60 seconds.

Place the soaked slices in the hot skillet and cook for 4-5 minutes per side, until deep golden brown and the filling is warmed through. The toast should feel firm when gently pressed in the center.

Repeat with remaining butter and bread slices. Serve immediately topped with powdered sugar, the reserved fresh blueberries, and warm maple syrup.