Preheat oven to 350°F (175°C) and grease a 9-inch pan.
In a bowl, whisk together gluten-free flour, sugar, baking powder, baking soda, and salt.
In another bowl, mix sour cream, eggs, butter, and vanilla until smooth.
Gradually combine the wet and dry ingredients, stirring gently.
Pour the batter into the prepared pan.
In a small bowl, mix sugar, gluten-free flour, and butter, then sprinkle on top.
Bake for 35-40 minutes, or until a toothpick comes out clean.
Allow to cool before serving.