Preheat oven to 350F degrees. Spray a 9-inch square baking pan with coconut oil and set aside.

In a medium bowl, whisk together flour, cocoa, salt, and baking powder. Set aside.

With an electric mixer, mix butter and sugar at medium speed for 2 to 3 minutes, or until combined and creamy. Add eggs, vanilla, and coffee. Continue beating until incorporated.

Scrape down sides of the bowl, and set the mixer speed to low. Then slowly add the flour mixture to the bowl, followed by the yogurt, and mix gently until combined. Stir in chocolate and walnuts if using.

With a spatula, spread mixture evenly in pan, and top with chocolate chips and any other toppings. I did walnuts and coconut flakes.

Bake for 20 to 25 minutes, or until a toothpick inserted in center of pan comes out dry. Cool on a wire rack before removing from pan and slicing into 16 squares.