Preheat the oven to 350 degrees F. Spray a 9x13 inch baking dish with nonstick baking spray.

In a small bowl, whisk together the flour, sugar, cocoa powder, cinnamon and salt. Add the butter pieces and using your hands, mix together the flour and butter until the flour is incorporated and in tiny pieces. The mixture will start to look like sand!

In another small bowl, whisk together the flour, baking powder, salt and soda.

In a large bowl, melt the butter. I just do this in the microwave until melted, in 30 second increments. Let cool for a few minutes.

Whisk in the sugar until combined. Whisk the eggs into the sugar and butter mixture. Whisk in the vanilla extract.

Add half of the dry ingredients, mixing until just combined. Stir in the greek yogurt until combined. Stir in the remaining dry ingredients until combined.

Spread half of the mixture into the baking dish. Sprinkle on half of the crumb. Spoon the remaining mixture on top of the crumb and gently use a spatula to spread it over the crumb. Top with the remaining crumb!

Bake for 45 to 50 minutes, or until a tester inserted in the center comes out clean.

Let cool completely before slicing and serving.