Adjust the oven rack to the 2nd level mark (just above center) and preheat your oven to 350ºF. Spray an 8-inch square baking pan with a flour-based baking spray. Set to the side.
In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set to the side.
1 cup All-Purpose Flour, 1 teaspoon Baking Powder, ⅛ teaspoon Salt
Using a hand mixer or stand mixer fitted with the paddle attachment, beat the eggs, egg yolk, granulated sugar, vegetable oil, and vanilla extract together on medium-high speed until fully combined and frothy.
2 Large Eggs, 1 Large Egg Yolk, 1 cup Granulated Sugar, 1 tablespoon Vegetable Oil, 1 teaspoon Vanilla Extract
Meanwhile, in a small saucepan, heat the semi-sweet chocolate chips and unsalted butter together over medium heat until fully melted and smooth. Remove from heat and immediately whisk or stir in the unsweetened cocoa powder and espresso powder.
6 tablespoon Unsalted Butter, 1 cup Semi-Sweet Chocolate Chips, ¼ cup Unsweetened Cocoa Powder, 1 teaspoon Espresso Powder
Carefully pour the melted chocolate mixture into the wet ingredients and mix until the chocolate is just combined.
Add in the whisked dry ingredients and mix on low speed until just combined. Then add in the chopped walnuts. You may also use a spatula to fold in both the dry ingredients and chopped walnuts.
1 cup Chopped Walnuts
Transfer the brownie batter to the prepared 8-inch baking pan. Spread out evenly in the pan, then top with additional chopped walnuts and sea salt flakes, if desired.
Bake for 25-28 minutes at 350ºF or until the top has a shiny crackly top to it.
Sea Salt Flakes
Remove from the oven and allow to cool completely before cutting into squares.
Cooled brownies can be kept at room temperature covered with aluminum foil overnight or can be stored in the refrigerator in a well-sealed container. Separate out the layers with wax or parchment paper if layering brownies in the container.