In a medium-sized bowl, combine the butter, brown sugar, yogurt, mango puree or mango baby food, vanilla extract, and the beaten egg.
In a separate bowl, combine the nutmeg, flour, flaxseed meal, baking powder, and baking soda. Stir this mixture into the other ingredients.
Line a muffin tin with paper baking cups. Scoop some of the mixture into each cup (about two-thirds full). Bake them for 15 to 20 minutes, until the muffins are golden brown on top.
Remove the muffins from the tin. Let them cool on a cooling rack, then enjoy!