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Preheat oven to 375 degrees. (190C).
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Place rack in top third of oven.
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Place muffin cups on the trail.
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Melt butter in a small saucepan over moderately low heat, then remove from heat.
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Whisk in milk, then whisk in whole egg, yolk, and vanilla until combined well.
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Whisk together flour, sugar, baking powder, cocoa powder, and salt in a bowl, then add milk mixture and stir until just combined. Notice: NEVER over mix the ingredients, for muffins, the best way is to mix by hand with a fork :).
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Cut banans into small cubes, and then fold them in and mix slightly.
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Divide batter among the muffin cups, spreading evenly. I tend to fill the cups a bit full so that the muffins will get over the cup top after baking - just like what we get from the shops!
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Bake about 18 to 20 minutes, or until a toothpick inserted in the center of a muffin comes out clean.
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Open the oven a bit to let the muffins settle before move out. Then cool in pans on a rack 15 minutes.
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Serve warm or at room temperature.