Prepare steamer and bring water to a boil. Wrap the steamer lid with a cloth to prevent water droplets from dripping onto the cake.
Grease a round or square heatproof pan and line the bottom with parchment paper if desired.
In a bowl, sift together flour, baking powder and baking soda. Set aside.
In another bowl, whisk oil and sugar until well combined.
Add eggs one at a time and mix well.
Stir in mashed bananas until smooth.
Gradually fold dry ingredients into wet mixture until just combined. Do not overmix.
Pour batter into prepared pan.
Top with banana slices and sprinkle Biscoff crumbs.
Steam over medium heat for 35–45 minutes, or until a skewer inserted in the center comes out clean.
Remove from steamer and let cool before slicing.