Preheat oven to 350 degrees F.

Grease an 8-inch square baking dish with cooking spray or butter. (I used my individual blondie pan and it made exactly 12.) You can also line with parchment paper.

In a medium bowl, whisk together the flour, baking powder, soda, salt, and cinnamon. Set aside.

In a medium saucepan, brown 1 stick of butter. You'll know it's close to done when it starts to foam up and become a light brown color. Take off the heat and add sugar and molasses. Whisk until combined. Let cool slightly, then add egg and mix well. Add vanilla extract and mix again.

Add the dry ingredients (and nuts, if using) all at once and fold in gently. Do not over mix- the batter should be pretty thick.

Spread in prepared baking dish and bake 15-18 minutes, or until very lightly browned on edges. Try not to overcook! Let cool completely before frosting.

For the frosting- in a small bowl whisk the powdered sugar. Brown 4 tablespoons of butter in a small saucepan. Add to sugar and whisk until well incorporated! Add milk, one teaspoon at a time, until you reach the desired consistency. Spread or spoon over cooled blondies. If using a blondie pan, you can cut edges off (I like the crunchier sides personally). If using an 8x8 square pan, cut into squares or triangles on a cutting board. Enjoy!