Make the caramel syrup: in a small saucepan over medium heat, combine 1/2 cup light brown sugar, 1/2 cup heavy cream, and 1/2 cup light corn syrup; stir and heat until the sugar is fully dissolved. Remove from heat and stir in 1 teaspoon vanilla (and an optional pinch of fine sea salt).
Slice the loaf: cut the cinnamon swirl bread into 8 even slices about 3/4 to 1 inch thick; keep slices unsoggy by handling gently.
Prepare the egg batter: in a large bowl whisk together 6 large eggs, 1/2 cup milk, 1 teaspoon ground cinnamon, 3/4 teaspoon salt, 1 1/2 teaspoons vanilla extract, and 1 tablespoon light brown sugar until smooth and thoroughly combined.
Heat the cooking surface: preheat a large griddle to 325°F or warm a large skillet over medium heat and add 1 tablespoon of the unsalted butter to melt.
Dip and cook the first batch: dip each bread slice into the egg mixture, coating both sides and letting excess drip off. Place slices on the hot griddle or skillet without overcrowding (leave about 1 inch between slices). Cook until golden brown, about 3–4 minutes per side.
Cook remaining slices: wipe the pan if needed and add the remaining 1 tablespoon butter before cooking the second batch; repeat dipping and cooking until all slices are done.
Make the whipped cream: pour 1/2 cup chilled heavy cream and 2 tablespoons powdered sugar into a cold bowl and whip on high until soft to medium-stiff peaks form (about 1–3 minutes). Stir in 1/2 teaspoon vanilla and an optional tiny pinch of salt.
Serve: transfer French toast to a wire rack or plates and serve immediately with warm caramel syrup, a dollop of whipped cream, and sliced strawberries or mixed berries.