Grease donut pans:

Coat donut pans with a neutral oil, unflavored oil, or butter using a pastry brush.

Preheat oven to 350°F:

Heat oven to 350°F (175°C).

Whisk milk mixture:

In a liquid measuring cup, combine 1 cup milk, 1 egg, and brown butter; whisk until fully blended.

Mix dry ingredients:

In a separate bowl, whisk together all-purpose flour, granulated sugar, salt, baking powder, baking soda, and cinnamon.

Combine wet and dry:

Pour the milk mixture and mashed banana into the dry ingredients; stir just until incorporated.

Fill donut cavities:

Fill each donut cavity with batter to about two-thirds full.

Bake until done:

Bake for 10-12 minutes until a toothpick inserted comes out clean. If still moist, extend baking by 2-3 minutes.

Cool donuts on rack:

Let donuts cool in the pan for 5 minutes, then move to a wire rack to cool entirely.

Prepare almond glaze:

For the glaze: mix confectioners' sugar, almond extract, and 2-3 tbsp milk until smooth.

Glaze each donut:

Submerge each cooled donut in the glaze, allowing excess to drip back.

Set glaze and garnish:

Allow glaze to set for several minutes, then garnish with sliced almonds if preferred.