Preheat your oven to 350°F (175°C) to ensure even baking.
Line your muffin tin with parchment paper liners or grease it with butter to prevent sticking.
In a large mixing bowl, combine the softened butter and ripe bananas. Whisk until smooth. Add the eggs and vanilla extract, whisking until well combined.
Fold the grated carrots into the wet mixture until evenly distributed.
In another bowl, mix almond flour, ground flaxseed, baking powder, cinnamon, and powdered sweetener until well incorporated.
Gradually add the dry mixture to the wet ingredients. Use a spatula to fold until just combined, being careful not to overmix.
Gently fold in the walnuts if using.
Spoon the batter into the prepared muffin tin, filling each cup about ¾ full.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Remove muffins from oven and let cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.
For the optional cream cheese frosting: beat cream cheese until smooth, gradually add powdered sweetener and vanilla extract, mix well, then spread or pipe onto cooled muffins.