Preheat oven to 350 and grease a loaf pan.
In a small bowl add the ground flaxseed and 2 tbsp of water and let set for 3-5 min.
In a larger bowl mash the bananas until smooth. Add in the sugar, butter, peanut butter, and flax eggs. Mix till smooth.
Fold in baking powder, baking soda, flour, salt, and milk. Mix until well combined.
Chop up the peanut butter cups and fold into the batter. Then pour the batter into the loaf pan.
In a separate bowl mix together the crumble ingredients until well combined.
Sprinkle the crumble on the top of the bread and gently press down. Place the bread in the oven on the middle rack. Let cook for about 50-60 min until set. Be sure to check on the bread at 30-40 minutes to be sure the crumble is not burning. If the crumble is brown then just place a piece of aluminium foil on top of the bread. Let the bread finish cooking until set.
Let the bread cool for at least an hour and ready to serve.