If your bananas aren’t already very ripe, place them on a baking sheet lined with parchment paper. Bake for 15-20 minutes, until they are nearly black. Be careful to not overcook or the bananas will leak.
Let the bananas cool to room temperature. Increase the oven temperature to 350°F (180°C).
Peel the bananas, then place them in a large bowl. Mash bananas using a hand mixer or fork.
Add the egg, vegetable oil, sugar, milk, and vanilla. Beat until combined.
Sift the flour, baking powder, baking soda, and salt into the banana mixture. Beat until just combined, being careful to not overmix.
Use a rubber spatula to gently fold the chocolate sprinkles into the batter.
Grease a 12-cup muffin tin with nonstick spray. Divide the batter among the cups.
Bake for 20-25 minutes, until the tops are lightly browned and a toothpick inserted in the center of a muffin comes out clean.
Place the chocolate chips in a heatproof liquid measuring cup. Pour over the hot heavy cream. Let sit for 1 minute, then stir until the chocolate is melted and smooth.
While the muffins are still warm, use the back of a wooden spoon to push down a hole into the center of each muffin. Allow to cool to room temperature.
Fill the muffins with the chocolate ganache.
Chill the muffins in the fridge for at least 1 hour, until the center has set.