Use a large baking tray; grease the tray. Preheat oven to 220 degrees C. Spoon choux pastry onto tray in 5 equal mounds, spacing them well apart. Make the mounds about 7 1/2 cm across and 3cm high. Bake buns for 18–20 minutes or until they are well risen and golden brown.
Remove tray from oven. With a skewer or the point of a small knife, make a small hole in the side of each bun. Return tray to the oven and cook for a further 5 minutes. Transfer buns to a wire rack to cool.
For the filling, tip beans into a bowl and mash with a potato masher until quite smooth. Cut the avocados in half; remove stones and scoop flesh into another bowl; mash roughly with a fork. Stir into beans with the lime juice.
Add tomato, onion, cucumber, garlic and coriander; season with salt and cayenne pepper to taste. Mix well.
Just before serving, split open buns and fill with guacamole mixture. Serve buns garnished with lettuce leaves.