Preheat and prep the pan. Preheat the oven to 350 Fahrenheit and grease a silicone donut pan with pan spray. Place the prepared donut pan on a baking sheet so it’s easier to move in and out of the oven.
Mix the wet ingredients. In a large mixing bowl, add the eggs, coconut sugar, vanilla extract, oil, and milk. Beat until smooth and fully combined.
Add the dry ingredients. Add the almond flour, baking powder, protein powder and salt to the wet ingredients. Using a silicone spatula, fold the dry ingredients into the wet ingredients until fully incorporated.
Remove clumps without overmixing. If you have a few clumps, gently whisk just until the batter is smooth, being careful not to overmix the donut batter.
Fill the donut molds. Pour or pipe the batter into the donut cavities. A piping bag is the easiest way, but a ziplock bag works great too. Note: if you use a metal mold pan, make sure to grease before filling with batter.
Bake. Bake for about 15 minutes, or until a toothpick comes out clean.
Prep the frosting. While the donuts bake, combine sugar, flour, and salt in a medium-sized saucepan and whisk well to combine and remove any lumps from the flour.
Turn on the heat. Turn stovetop heat to medium and whisk in milk until mixture is smooth.
Whisk until thickened. Continue whisking constantly until mixture is thickened to a pudding-like consistency. Be patient and do not turn up the heat to avoid burning the sugar. This will take about 3 minutes.
Cool. Remove from the heat and transfer to a heat proof container. Cover the surface with plastic wrap to avoid a skin forming. Allow to cool to room temp.
Make frosting. Once the mixture is cooled, use an electric mixer to beat the softened butter on high until fluffy. Lower the speed to medium and gradually spoon in the flour mixture beating until well combined, scraping down the bowl with a spatula if needed. Continue whipping until light and airy.
Frost the donuts. Frost each donut with the banana frosting and top with fresh banana slices.