Preheat oven to 350 degrees F. Liberally grease a 13x9" baking pan with cooking spray and set aside. In a large bowl, melt the butter and chocolate together. You can either do this in a microwave-save bowl in the microwave, heating at 45-second increments, stirring after each round until smooth, or you can do it in a glass bowl over a pot of simmering water, like a double-boiler method (just make sure the water is not touching the bottom of the bowl). I did the microwave method and it worked just fine.
NOTE: if you are using baking bars (such as Baker's), break them up into individual, segmented pieces before melting. This helps expedite the melting process and make for a smooth batter.
In the bowl of a stand mixer, beat together the sugar, eggs and vanilla with the paddle attachment until creamy, about 2 minutes. Add in the chocolate butter mixture until smooth. Then, add in the flour, espresso powder (if using) and salt until a soft yet thick batter has formed.
Spread the batter evenly into the prepared pan. Bake for approximately 35-45 minutes or until a toothpick inserted near the center comes out with moist crumbs (not super wet). Here's where it's up to your preference. I baked mine for exactly 37 minutes, creating VERY fudgy brownies that almost seemed slightly undone which is how I like it. If you want something still fudgy but more structured, bake up to 45 minutes. Do not overbake, as brownies WILL continue to cook and set up once they cool.
Allow brownies to cool, preferably overnight, before cutting into bars. Brownies may slightly sink in the middle during cooling; this is normal. If you'd like, you can frost them or sprinkle them with powdered sugar. Serve room temperature or warm, and store leftovers airtight at room temperature for up to a week. These would freeze well, too - wrap individually and freeze up to 3 months.