Heat oven to 350ºF. Line a 12-cup muffin tin with paper liners.
In a large mixing bowl, use an electric beater to beat the egg white until soft peaks form. Set aside.
In a second mixing bowl, cream together the coconut oil and sugar until it resembles fine, wet sand. Beat in the egg yolk. Stir in the bananas, cashew milk, and vanilla.
Whisk together the flour, baking powder, baking soda, and salt. Beat the dry ingredients into the wet ingredients to make a thick batter; gently fold in the whipped egg white.
Divide the batter among the prepared muffin cups. Bake for 18-22 minutes or until the tops of the cupcakes are gently domed and light golden brown.
Remove from the oven and let cool completely.
While the cupcakes cool, prepare the frosting: Use an electric mixer to beat the shortening until it's light and creamy. Add the sugars and vanilla and beat until the frosting is light and fluffy. If desired, beat in up to two teaspoons of cashew milk to thin out the frosting.
Frost cupcakes after they've completely cooled.