1Using electric mixer, beat butter and both sugars in large bowl until well-blended. Add egg and vanilla and beat until fluffy. Beat in flour, salt and baking soda. Stir in butterfingers (dough will be moist). Chill cookie dough at least 30 minutes and up to 1 hour.
2. Preheat oven to 350°F. Line baking sheets with parchment paper. Using 1 rounded Tablespoon of dough for each cookie (I use a small cookie scooper), drop cookie dough in mounds onto prepared baking sheets, spacing 3 inches apart (about 8 cookies per sheet; cookies will spread). Bake cookies until golden brown, about 12 minutes. Transfer parchment with cookies to rack and cool completely. (Can be prepared 4 days ahead. Store in airtight container.)