Preheat oven to 350ºF; line 2 baking sheets with baking paper, lightly coated with cooking spray. I used Silpats! Lightly spoon flour into a dry measuring cup; level with a knife. Sift together flour, 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 tablespoons 2 inches apart onto prepared baking sheets.

Bake for 8 to 10 minutes or until almost set. Cool on pans 2 to 3 minutes or until firm. Remove cookies from pans; cool on wire racks.