Preheat oven to 350F. Lightly grease two 6-cup donut pans. Set aside.
Make the donuts: In a large bowl, whisk mashed banana, oil, sugar, yogurt, espresso, milk, egg, and vanilla extract until smooth. Gradually fold in flour, cocoa, baking powder, and salt until batter is smooth.
Fill prepared donut pans 3/4 full of batter and smooth tops. Bake donuts at 350F 14-15 minutes or until donuts spring back when lightly touched. Cool donuts in pan 10 minutes, then transfer to a wire cooling rack.
Make the glaze: In a small saucepan, bring espresso, cream, and agave to a low simmer. Place chocolate chips in a small bowl, then pour hot cream mixture over top. Gently whisk chocolate until melted and glaze is completely smooth. Whisk in vanilla extract.
Dip cooled donuts into glaze and place on wire cooling rack to set. Enjoy!