Preheat oven to 350 degrees.

Lightly spoon flour into a dry measuring cup; level with knife. Combine flour, baking soda and salt; set aside. Melt butter in a large saucepan over medium heat. Remove from heat; stir in cocoa powder and sugars. (Mixture will resemble coarse sand.) Add yogurt and vanilla, stirring to combine. Add flour mixture, stirring until moist. Drop by level tablespoon 2 inches apart on lightly greased baking sheets.

Bake for 8-10 minutes or until almost set. Cool on pans 2-3 minutes or until firm; remove and transfer to wire racks.