Preheat the oven to 180°C (350°F). Line a 12-cup muffin tin with liners and grease 10 of the cups.
In a large bowl, beat the softened butter and allulose until creamy, then add salt (if using), vanilla extract, and eggs; beat until well combined.
Stir in the milk to the wet mixture until combined.
In a separate bowl, sift together the coconut flour and baking powder to remove lumps.
Fold the dry ingredients into the wet ingredients and mix until a uniform batter forms. Add a little more milk if the batter seems too thick.
Divide the batter evenly among the 10 prepared muffin cups, filling each about three-quarters full.
Bake on the center oven rack for 17–20 minutes, or until a skewer inserted into the center of a cupcake comes out clean.
Let the cupcakes cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before frosting.
Frost the cooled cupcakes with the keto vanilla frosting.