Preheat the oven to 350°F. Spray two baking sheets with cooking spray (or line with silpat mats or parchment paper).
In a medium bowl, whisk together the flour, baking soda and salt.
In a medium saucepan, melt the butter. Remove from heat and then whisk in the cocoa and sugars (it's will be the texture of beach sand). Then whisk in the yogurt and vanilla, stirring to combine. Stir in the dry ingredients, stirring until moist.
Drop by level tablespoons 2 inches apart on the prepared baking sheets. Bake 8 to 10 minutes or until cookies are almost set. Cool cookies on pans until set, then transfer to a wire rack to cool completely.