Preheat oven to 350 degrees. Line a muffin pan. Set aside.
Make the frangipane filling by adding all the ingredients into a medium mixing bowl. Stir with a spatula until well combined. It will be like a thick, smooth paste.
In a large bowl, make the flax egg by combining the ground flaxseed and water. Stir and set aside for 5 minutes to thicken.
In a medium bowl, add the flour, cinnamon, baking soda, baking powder, and salt. Whisk to combine.
In the large bowl with the flax egg, add the bananas and mash them until smooth. Add the oil, sugar, milk, vanilla, and apple cider vinegar. Whisk until well combined.
Slowly add the dry ingredients (1/3 of the dry at a time) to the wet and stir with a spatula until just combined and you don’t see anymore flour streaks in the batter. Don’t overmix because you don’t want a tough banana bread.
Start to layer your batter, frangipane, batter and frangipane. Fill each muffin tin with 1.5 Tbsp of batter. I filled about 11 cups. Then take about a heaping 1 tsp of frangipane and place on top of the batter filled cups. Place another 1.5 Tbsp of banana muffin batter on top of the frangipane. Place another 1.5-2 tsp of frangipane on top, evenly spreading it over the top (it doesn’t have to be perfect). Sprinkle sliced almonds on top.
Bake for 25-28 muffins, or until a toothpick inserted in the center comes out clean. These are big muffins! If yours are a little smaller, bake for less time. Let it cool in the pan for 5 minutes. Transfer to a wire rack to cool completely. Sprinkle powdered sugar on top. Enjoy!
Store leftovers in an airtight container. The tops will lose their crisp when stored but it will still be delicious!