Preheat oven to 160°C. For the crumble, combine oats, sugar, baking powder, salt and cinnamon in a bowl. Rub butter in with your fingers until it resembles breadcrumbs. Stir in the egg white. Spread mixture thinly onto a baking paper-lined baking tray. Bake for 10 minutes or until dark golden. Cool. Whiz it in a food processor to a fine crumb. Divide the creme de cacao, coffee and mascarpone between two cocktail shakers filled with ice. Shake each until combined. Divide among the glasses. Sprinkle with cinnamon crumble to serve.