In a medium bowl, mix the softened cream cheese, powdered sugar, and vanilla extract until smooth.
Spread the cream cheese mixture evenly on 4 slices of bread, then top with the remaining 4 slices to create sandwiches.
In a shallow dish, whisk together the eggs, milk, and ground cinnamon until well combined.
Dip each sandwich into the egg mixture, allowing them to soak for a few seconds on each side.
In a large skillet, melt the butter over medium heat.
Cook the sandwiches for about 3-4 minutes on each side, or until golden brown and cooked through.
Remove from the skillet and keep warm.
For the blackberry sauce, combine the blackberries, granulated sugar, lemon juice, and water in a small saucepan over medium heat.
Cook for about 5-7 minutes, stirring gently until the blackberries break down and the sauce thickens slightly.
Serve the stuffed French toast warm, drizzled with the blackberry sauce.