Preheat oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners. Whisk gluten-free flour, baking powder, baking soda, and salt in a bowl.

Stir almond milk and vinegar together and let sit briefly. In a large bowl, whisk sugar, melted coconut oil, eggs, and vanilla until smooth, then whisk in the milk mixture.

Add dry ingredients to wet and fold with a spatula until no flour streaks remain. Batter will be thick.

Divide batter evenly into liners and bake 18 to 20 minutes, until a toothpick comes out clean. Cool in pan 5 minutes, then transfer to a wire rack to cool completely.

Beat dairy-free butter until creamy. Gradually add powdered sugar, vanilla, and 1 tbsp almond milk, then beat until fluffy, adding more milk as needed. Frost cooled cupcakes and serve.