Preheat the oven to 350 F
In a large bowl, whisk together the mashed banana, eggs, sugar, oil, milk and vanilla extract until smooth.
Add in the oat flour, almond flour, salt and baking soda and mix until well combined.
Divide the batter equally into bowls.
Whisk the 3 tbsp cocoa powder with the 3 tbsp of hot water then pour it into one of the bowls to make the chocolate layer. Mix the chocolate chips into the chocolate bowl.
Line a 12-count muffin tray with paper liners. Alternate between the two batters by scooping 1 tbsp of each into each muffin cup. Repeat until all the cups are filled. If there is any remaining batter, top any of muffins with a spoonful. Swirl the batter gently using a toothpick to create a marble look.
Bake the muffins for 20-22 minutes, until the tops are firm to touch.
Let cool completely. Keep stored in an airtight container for up to 4 days, or refrigerate for up to a week.