Preheat oven to 350°F. Line a 9x5-inch loaf pan with parchment paper, leaving about 1-inch overhang along the sides.
First mash the bananas on a flat surface with a fork and measure out 1 ½ cups (you will need 360g of banana flesh which is about 3 medium bananas). Place mashed banana in a large mixing bowl. Add sugar and whisk until glossy and syrupy. Whisk in eggs, oil and vanilla until blended. Whisk in baking powder, baking soda, salt and spices. Add flour and fold it in until just combined. Reserve 1 cup of batter and transfer it to another medium bowl.
Melt chocolate until smooth over a double boiler or in the microwave in 20 second increments with frequent stirring. Add it to the bowl with reserved banana batter with 1 tablespoon of milk and mix to combine evenly.
Dollop large spoonfuls of the regular banana batter alternately with the chocolate batter into prepared pan then run a knife around back and forth 5 or 6 times to swirl the batters together.
Bake for 45-55 minutes until the top is evenly browned and a skewer inserted into the center comes out clean. Let cool for 10 minutes, then turn out onto a wire rack to cool completely.