Use an electric mixer to beat together the softened mascarpone cheese, maple syrup and vanilla extract until smooth.
In a large bowl, whisk together the eggs, half and half, cinnamon and vanilla, then set aside.
Spread about 2 tablespoons of filling on a slice of bread, leaving roughly a ½ inch border around the edges. Top with another piece of bread and gently press to secure. Repeat with the remaining bread and filling.
Dip one sandwiched piece of bread into the custard, turning a few times to thoroughly coat, then repeat with another. Let them sit in the custard while you melt 1 tablespoon butter in a large skillet over medium-low heat.
Transfer the custard soaked bread to the hot pan (you should be able to fit two in the pan at once, if not, cook one at a time) and slowly cook until golden brown and crisp, about 3-4 minutes, then flip and cook for an additional 3-4 minutes on the other side. Be sure to keep the heat at a relatively low temperature to avoid burning.
Melt more butter in the pan and repeat with the remaining pieces.
Slice each piece of french toast in half on a diagonal, then place 3 halves on each plate. Top with berries and serve with maple syrup on the side