Preheat the oven to 160°C (320°F).
Cream the soft butter with the muscovado sugar, brown sugar, and vanilla sugar until light and fluffy, about 2 minutes.
Mix in the liquid vanilla and eggs one at a time, beating well after each addition.
Mash the bananas with a fork until roughly pureed and fold into the batter.
In a separate bowl, mix the flour, baking powder, and chocolate chips. Gently fold into the banana mixture until just combined.
Pour the batter into a greased and floured loaf pan, or lined with parchment paper.
To ensure even rising, dip a knife in oil or melted butter and draw a line along the center of the batter.
Bake for about 50 minutes, or until a knife inserted in the center comes out clean.
If desired, brush the top of the cake with a syrup made from boiling 30g of water with 30g of sugar, then apply it with a brush while the cake is still hot. Optionally, wrap the hot cake in cling film to retain moisture.
For decoration, melt 40g of chocolate and drizzle it over the cake in a zigzag pattern using a piping bag or a cut corner of a ziplock bag.
Let the cake cool or serve warm with melting chocolate chips.