Preheat oven to 180°C. Line 15 holes of two 12-hole 80ml (1/3-cup) muffin pans with muffin cases.
Place sultanas and 2 cups (500ml) boiling water in a heatproof bowl for 5 minutes to soften. Drain and rinse under cold water.
In a separate bowl, whisk oil, 2/3 cup (165ml) maple syrup, eggs, mashed banana, milk and sultanas until combined. In a separate bowl, combine flour, baking powder, vanilla seeds, allspice and 1/2 teaspoon salt flakes. Add wet ingredients to dry ingredients and stir to combine. Divide batter between prepared pans. Sprinkle with walnut and chopped banana. Bake for 25 minutes or until a skewer inserted into a muffin comes out clean.
Meanwhile, place remaining 1/3-cup (80ml) maple syrup in a saucepan over medium heat and bring to the boil. Drizzle over muffins as they come out of oven. Set aside to cool before serving.