Mix brown sugar and cinnamon in small bowl. Set aside.
Cut challah bread into ½-inch slices. Brush each slice with butter.
Sprinkle brown sugar mixture on top of each slice, using approx. 3/4 of mixture. Set rest of mixture aside.
Place bread slices on baking sheet and bake for 10 minutes. Remove and let cool on wire rack.
Vanilla Sauce
Beat eggs, egg yolks, and sugar in stand mixer at medium speed for 3 minutes or until light and fluffy.
Add vanilla extract. Transfer to medium bowl. Set aside.
Pour milk into medium saucepan. Scrape vanilla bean seeds into milk using a paring knife or butter knife.
Heat mixture on medium low until it is almost boiling.
Slowly pour milk mixture into the egg mixture, stirring continually with a wooden spoon.
Pour the mixture into medium saucepan and heat on low, stirring with a wooden spoon until the vanilla sauce has thickened slightly. Remove from heat and set aside.
Bread Pudding Assembly
Butter or spray 9 x 13" pan with nonstick cooking spray. Transfer baked bread slices to pan.
Pour milk mixture on top of bread slices and sprinkle with remaining brown sugar mixture.
Bake for 16-21 minutes.
Open the oven and pour the heavy cream on top of the bread pudding. Cook for another 9 minutes or until golden brown.