Preheat oven to 350F, placing one rack in the upper third and one rack on the lower third of the oven. Line two baking sheets with parchment paper. Combine flour, baking soda and salt in a small bowl, and set aside.
Melt butter in a medium saucepan over medium heat sizzling. Remove from heat and whisk in cocoa until smooth. Add both sugars and stir until blended. It’ll be stiff and sandy but don’t fret! Stir in yogurt and vanilla. Add dry ingredients, mixing until just combined.
Scoop rounded tablespoons of dough onto sheets, about 1 1/2 inches apart. Bake 10-12 minutes, until cookies look dry on top and are cracked. For even baking, rotate sheets top to bottom and front to back halfway through. Cool before serving.