Make muffins: Preheat oven to 180°C. Line a 6-hole Texas muffin tin. Mash bananas in a bowl, then whisk in oil, sugar, eggs, and vanilla. Add flour, baking soda, and salt; whisk smooth. Fold in ¾ of the chocolate. Divide batter into tins, top with remaining chocolate, and sprinkle each with 1 tsp raw sugar. Bake 20–25 minutes until golden.
Make filling: Whip egg yolk, sugar, and vanilla until pale. Add mascarpone and salt; beat until smooth. Transfer to a piping bag.
Assemble: Mix espresso with liqueur (optional). While muffins are warm, cut a small hole in the centre, pour in 2 tsp espresso mix, then pipe in mascarpone. Add a small dollop on top and dust with cocoa.
