The Muffins:
Preheat the oven to 375 F (190 C).
Butter muffin molds or line them with paper cups.

In a large bowl, combine the flour, baking powder, sodium bicarbonate, and chocolate chips.

In a medium bowl, beat the milk, eggs, melted butter, and grated zest.
Add the banana puree and mix well.
Pour over the dry ingredients and stir just enough to moisten.

Fill the muffin molds to the top, rounding them slightly.
Bake in the oven for 20 to 25 minutes, or until the top of the muffins is firm to the touch.

The Glaze: Put all the glaze ingredients in a saucepan and bring to a boil.
Cook until the mixture has thickened slightly, about 2 minutes.

Brush the muffins with the glaze once they have cooled.