Preheat oven to 350 degree F. To make oat crisps, in a medium bowl combine melted butter, brown sugar, 2 tablespoons granulated sugar, and corn syrup. Stir in oats. Place tablespoonfuls several inches apart on baking trays lined with parchment. Bake about 12 minutes. Let cool on tray; store in an airtight container until ready to serve. (Can be made a day in advance.)
For sauce, mix cocoa powder with 1/2 cup water to make a thick paste. In a heavy-bottomed saucepan, combine remaining water and 1 cup sugar. Bring to a boil; cook until sugar dissolves. Stir in cocoa paste. Stirring occasionally, simmer for 10 minutes until sauce coats the back of a spoon. Refrigerate and reheat in a microwave before serving. (Can be made several days in advance.)
You can buy creme fraiche pre-made. Or make your own: In a small saucepan, combine buttermilk and cream; cook on low heat, until mixture reaches 85 degrees F. Set aside at room termperature until mixture thickens (5-8 hours). Refrigerate until ready to use. (Can be made 1 week in advance.)
To assemble: slice bananas. Place an oat crisp on each plate and top with a few banana slices. Spoon cocoa sauce over banana slices. Place a dollop of creme fraiche over cocoa sauce. Top with another oat crisp, and repeat layers. Serve immediately.