Make the namelaka

Pour 1 1/2 tablespoons cold water into a small bowl, and sprinkle evenly with gelatin; let stand 5 minutes. Bring milk to a low simmer in a small saucepan over medium. Remove from heat, and whisk in gelatin mixture until dissolved. Add chopped chocolate, and let stand for 1 minute; whisk until chocolate is melted and smooth. Whisk in vanilla and 2/3 cup heavy cream. Pour mixture through a fine wire-mesh strainer set over a large bowl; discard any solids. Press plastic wrap directly onto surface of namelaka; refrigerate until set and firm, at least 4 hours or up to 2 days.

Whisk remaining 1/2 cup heavy cream in a medium bowl until stiff peaks form, 1 to 2 minutes. Whisk namelaka until loosened and smooth, about 20 seconds; fold whipped cream into namelaka.

Make the sandwiches

Cut each banana crosswise into thirds. Spread about 1/3 cup whipped namelaka in an even layer on each of 4 bread slices; arrange 3 banana pieces diagonally over each slice, pressing banana pieces gently down into whipped namelaka.

Sprinkle espresso powder, if using, evenly over bananas; top with remaining whipped namelaka (about 1/3 cup per bread slice), and spread to create a flat surface. Top with remaining 4 bread slices, and press gently.

Wrap each sandwich tightly with plastic wrap. Refrigerate until filling feels firm, about 1 hour. To serve, unwrap sandwiches; using a serrated knife, cut off and discard crusts. Slice each sandwich in half diagonally (cutting banana pieces crosswise).