Preheat the oven to 325 F (160 C).
Line 8 jumbo muffin molds with paper liners.

Mix the flour, baking powder, salt, and tonka bean in a small bowl. Set aside.
In a bowl, mix the mashed bananas with the lemon zest, lemon juice, and rum. Set aside.
Melt the chocolate with 2 tablespoons of butter in the microwave, stirring every 10 seconds. Set aside.

Cream the butter with the sugars using an electric mixer until the mixture is white.
Add the eggs one by one, mixing between each addition.
Add the dry ingredients mixture in alternation with the milk, then add the banana mixture and mix well.

Separate the dough into two bowls, and mix the chocolate into one of the doughs.

Pour the doughs into the molds in alternating spoonfuls.
Bake for 1 hour or until a toothpick inserted into the center comes out clean.
Let cool for 15 minutes on a rack.