Preheat oven to 350°F. Use a nonstick baking spray and prepare an 8-inch square baking pan. Line with parchment, leaving an overhang on two sides, spray parchment and pan.
Melt butter in a small saucepan on medium heat, stirring occasionally. Butter will foam and start to become very fragrant. Cook until butter turns a nutty brown color. Remove from heat and pour through a fine-mesh sieve into a large bowl. Add honey to still warm butter and stir. Let cool to room temperature.
In a small bowl, combine flour, baking powder, and salt. Set aside.
Add brown sugar to bowl with butter and honey, whisk until smooth. Whisk in egg and vanilla.
Add dry ingredients, chocolate chunks, and optional walnuts. Stir until mixture is just incorporated. Do not over-stir.
Pour batter into pan and distribute evenly, smoothing out the top. Bake for 25 to 30 minutes. Test center with a toothpick, until it comes out clean.