Preheat oven to 350 degrees. Line a baking sheet with parchment paper.
In a large bowl, whisk together sugar, egg, sour cream, and vanilla extract until completely combined. Add softened butter.
In a separate bowl, whisk together flour, baking soda and salt.
Add dry ingredients to wet and stir to combine. Knead until it just comes together, then mold into a ball. Wrap in plastic wrap and refrigerate for 30 minutes.