In a medium bowl, beat the softened cream cheese, sugar, vanilla, and lemon zest until smooth and spreadable. Fold in the blueberries gently, taking care not to crush them.

Spread the blueberry cream cheese mixture evenly over 4 slices of bread, leaving about a 1/2-inch (1 cm) border around the edges. Top with the remaining 4 slices and press gently around the edges to help the sandwiches hold together.

Whisk the eggs, milk, maple syrup, vanilla, cinnamon, and salt in a shallow bowl or baking dish until smooth and evenly combined.

Let the assembled sandwiches stand for about 5 minutes before dipping. This gives the filling time to settle and makes the sandwiches easier to handle.

Heat a 10- to 12-inch nonstick skillet or griddle over medium-low heat. Add 1 tablespoon (14 g) of butter and let it melt until foamy but not browned.

Place one sandwich at a time in the custard. Soak for about 15 to 20 seconds per side, turning carefully. The bread should be moistened but still firm enough to lift without tearing.

Transfer the stuffed sandwiches to the skillet: Cook for about 3 to 4 minutes, until the underside is golden brown and the edges are beginning to set.

Slide a wide spatula fully underneath each sandwich and turn it gently. Cook the second side for another 3 to 4 minutes. Reduce the heat if the bread is browning too quickly.

Continue cooking until both sides are golden brown and the center is warm. The cream cheese should be soft and the egg custard fully set. If the exterior is browned but the center is still cool, reduce the heat to low and cook for another 1 to 2 minutes per side.

Transfer the finished French toast to a plate: Add the remaining 1 tablespoon (14 g) butter to the skillet and repeat with the remaining sandwiches if cooking in batches.

Cut each stuffed French toast sandwich in half if desired. Serve with optional maple syrup, powdered sugar, and fresh blueberries.