In a small, microwavable bowl, microwave the chocolate chips and cream for about 45 seconds; stir until smooth and cover to keep warm.
On a work surface, lay 12 wonton wrappers flat. Top 6 of them with 4 banana slices each, arranging the slices in the center. Brush the other 6 wrappers with some of the egg wash and place wet side down over the filled wrappers, pressing the edges to seal. Repeat with the remaining wrappers and banana slices.
In a large skillet, melt half of the butter over medium heat. Add half of the wontons and cook, turning once, until deep golden-brown, 2 to 4 minutes; drain on paper towels and wipe out the pan. Repeat with the remaining butter and wontons. Divide the wontons among 4 plates; drizzle with the chocolate sauce and dollop with the creme fraiche.