Preheat the oven to 350°F / 175°C. Butter and line an 8x4" loaf pan with parchment paper.
In a small microwave safe bowl, melt together the butter and 90g (½ c.) chocolate chips in 30 second intervals, stirring in between each interval.
Once the mixture is melted and smooth, whisk in the cocoa powder and set aside.
Add the egg and sugar into a medium sized mixing bowl. Whisk vigorously for at least 1 minute, until the mixture is light and fluffy.
Add in the melted chocolate mixture and gently mix until well combined.
Add in the flour, salt, and espresso powder (if using). Use a rubber spatula to lightly fold the flour into the ingredients. When there are only a few streaks of flour remaining, add in the remaining 45g (¼ c.) of chocolate chips. Fold just until no flour streaks remain.
Pour the brownie batter into the prepared pan. Bake for approximately 20 minutes, or until the brownies are set on the outsides and only ever so slightly jiggle in the middle.
Wait for the brownies to fully cool before cutting. Serve, and enjoy!