For the syrup: In a medium saucepan, bring the water and sugar to a boil over high heat, stirring until the sugar dissolves. Boil until the syrup reduces to 1 cup, about 10 minutes. Remove the pan from the heat and whisk in the cream and cinnamon. Keep the syrup warm over low heat until ready to serve. (The syrup can be made 1 day ahead. Cool, then cover and refrigerate. Reheat before serving).
For the French toast: Preheat the oven to 200 degrees F. Preheat a nonstick griddle or large nonstick saute pan over medium heat.
Using a serrated knife, remove the top from the panettone. Cut the bottom of the panettone in half crosswise. Cut each half into 4 equal pieces. In a large bowl, whisk together the eggs, cream, milk, and sugar until smooth. Melt 1 tablespoon of butter on the griddle. Working in batches, dip slices of panettone into the custard, turning to allow both sides to absorb the custard. Cook the soaked panettone slices until golden brown and firm to the touch, about 4 minutes per side. Transfer the French toast to a baking sheet and keep warm in the oven. Repeat with remaining 1 tablespoon butter and panettone slices.
Transfer the French toast to plates. Drizzle the cinnamon syrup over the French toast and place a dollop of mascarpone on top. Lightly dust with powdered sugar and serve with fresh berries.
The top of the panettone can be reserved and used for toast.