In a shallow bowl, whisk together 6 eggs, 1 cup whole milk, 2/3 cup heavy cream, 1/4 cup granulated sugar, 2 tsp vanilla extract, 1 tsp cinnamon, and 1/2 tsp salt until fully combined and smooth.
6 eggs, 1 cup whole milk, 2/3 cup heavy cream, 1/4 cup granulated sugar, 2 tsp vanilla extract, 1 tsp cinnamon, 1/2 tsp salt
Dip all 20 slices thick-cut brioche into the custard batter, letting each soak for about 20–30 seconds per side so the bread is well saturated but not falling apart.
20 slices thick-cut brioche
Melt 3 tablespoons of the 6 tbsp unsalted butterin a large skillet or griddle over medium heat. Cook the soaked slices in batches for about 2–3 minutes per side, until golden brown and cooked through. Repeat until all the bread is toasted. Transfer each back to a wire rack to cool completely.
6 tbsp unsalted butter
Once cooled, arrange all the french toast slices on a parchment-lined baking sheet and freeze for at least 1 hour until fully firm.
Take 2 of the frozen french toast slices and cut them into small ½-inch cubes. Keep them in the freezer until ready to use, they need to stay frozen so they don't melt the ice cream when mixing.
In a large chilled bowl, fold 2/3 cups maple syrup into 5 cups vanilla ice cream, softened until loosely swirled. Add the frozen french toast cubes and fold a few times until evenly distributed. Work quickly to keep everything cold. Transfer the bowl to the freezer for 30 minutes to firm back up.
2/3 cups maple syrup, 5 cups vanilla ice cream
Working quickly, scoop a generous amount of the maple French toast ice cream filling onto the flat side of one of the remaining 8 frozen French toast slices. Top with another slice, flat side down, and press gently to spread the filling to the edges.
Place the assembled sandwiches back on the parchment-lined baking sheet and freeze for at least 30 minutes to firm up before serving.
Dust with 2 tbsp powdered sugar if desired and serve immediately straight from the freezer.
2 tbsp powdered sugar