Preheat oven to 325° Fahrenheit. Spray a 13 x 9” rectangular baking pan with nonstick cooking spray. Set aside.
Place butter, espresso powder and chocolate chips in a medium saucepan over medium low heat. Whisk until mixture is completely melted and espresso has dissolved. Set aside.
In a medium sized mixing bowl, whisk together flour, cocoa and salt until well blended. Set aside.
In a large mixing bowl, whisk eggs, granulated sugar and vanilla until mixture turns pale yellow. This should take about 2 minutes.
Whisk chocolate mixture into egg mixture and blend well.
Using a rubber spatula, add dry ingredients into chocolate and egg mixture and blend until the batter is well combined.
Pour batter into prepared pan.
Scatter mini chips over the top of the batter. Sprinkle with flaky sea salt.
Bake for approximately 55 minutes for a brownie texture but reduce the time by 15 minutes if you would like the cake to be fudgier. Dust with confectioner’s sugar.