Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, allowing some overhang on the sides for easy removal later.
In a medium bowl, cream together 10 tablespoons of softened unsalted butter and 1 cup of granulated sugar until light and fluffy. Add in 1 tablespoon of espresso powder and 1 teaspoon of kosher salt, mixing until well combined. Gradually incorporate 1 cup of all-purpose flour, mixing just until the dough comes together.
Press the shortbread mixture evenly into the bottom of the prepared pan. Bake in the preheated oven for about 15 minutes, or until lightly golden. Remove from the oven and allow to cool slightly.
In a medium saucepan over low heat, combine 14 tablespoons of unsalted butter and 4 ounces of chopped unsweetened chocolate. Stir until melted and smooth. Remove from heat and let cool slightly.
In a large mixing bowl, combine 2 cups of granulated sugar and 1 tablespoon of pure vanilla extract. Add the melted chocolate mixture, whisking to combine. Then, add 3 large eggs one at a time, mixing well after each addition. In another bowl, whisk together 1 cup of all-purpose flour, 2 tablespoons of unsweetened cocoa, 1 teaspoon of kosher salt, and ½ teaspoon of baking powder. Gradually mix the dry ingredients into the wet ingredients until just combined.
Pour the brownie batter over the baked shortbread layer, spreading it evenly. Bake for an additional 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Allow the bars to cool completely in the pan before lifting them out using the parchment paper.
Once cooled, cut the Espresso Shortbread Brownie Bars into squares or rectangles. Enjoy them plain or dusted with powdered sugar for an extra touch!
