Preheat your oven to 350°F /175°C and grease an 8-inch round or square cake pan, or line it with parchment paper.
In a medium bowl, whisk together the oat flour, baking powder, baking soda, salt, cinnamon, and coconut sugar.
In another bowl, combine the applesauce, melted coconut oil, eggs, vanilla extract, and milk. Ensure all ingredients are at room temperature; otherwise, your coconut oil will harden. Whisk until smooth.
Gradually add the wet mixture to the dry, stirring just until combined. Avoid over-mixing, as it will make the cake less light and fluffy.
Pour the batter into the prepared cake pan and spread it evenly. Bake for 30 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.