Preheat the oven to 350 degrees. Coat a 9 x 13inch baking pan with cooking spray.
Melt the chocolate and butter in a double boiler or heatproof bowl set over a pot of barely simmering water, stirring occasionally.
In a medium bowl, whisk together the flour, cocoa, salt, and baking soda.
In a large bowl whisk the eggs and sugar until smooth. Add the yogurt, oil and vanilla and whisk to combine. Add the chocolate-butter mixture and whisk until blended. Add the flour mixture and mix until just moistened.
Transfer the mixture to the prepared pan and sprinkle with nuts if using. Bake for 20-25 minutes, until a wooden toothpick inserted in the center comes out with a few moist crumbs. Cool completely in the pan on a wire rack. Cut into 24 squares.
* whole grain pastry flour (also called whole wheat pastry flour) is found in many major supermarkets and in most health food stores. If you cannot find it you may substitute 1/2 cup whole wheat flour and 1/2 cup all purpose flour in this recipe.