Line a baking sheet (I used an 18 X 13 inch) with parchment.
In a mixing bowl, combine the brownie mix (1 box), butter (6 tablespoons), egg white (1), 1/3 cup water, and chocolate chips (1/2 cup) until evenly combined. Turn out the mixture onto the parchment-lined baking sheet. Use an offset spatula to spread the mixture evenly, about 1/4 inch thick layer.
Bake in the preheated oven until baked through and dry, 30-40 minutes. Turn the oven off, leaving the brownie bark in the oven, until the oven has totally cooled and the bark is dry.
Meanwhile, make the topping. Place the 3/4 cup semisweet chips in a microwave-safe bowl. Melt on high for 30-45 seconds. Stir chips to melt completely. Pour melted chips into a quart resealable bag.
Cut one corner and drizzle melted chocolate diagonally over brownie bark. Repeat and drizzle on the opposite diagonal. Sprinkle warm chocolate with mini semi-sweet chips (1/4 cup). Allow to cool and set. Break apart or use a pizza cutter to cut into irregular shapes.
Notes
Choose a chocolatey mix. A dark chocolate brownie mix gives you that deep, brownie-shop flavor. If you’re a serious chocolate lover, look for one that already includes chips and still add the extra 1/2 cup — no one ever complains about too much chocolate.
Take your time spreading. The batter is thick and studded with chips, so don’t rush this step. A small offset spatula really is your best friend here — work in small swoops, nudging the batter into an even layer and coaxing any clumps of chips into more open spots so every bite has a nice mix of chocolate.
Thin is your friend. If the layer is too thick, the middle will stay brownie-soft instead of crisping. When in doubt, keep gently pushing the batter outward. You should almost be able to see the parchment through it in a few spots.
Don’t skip the “cool in the oven” step. It might feel strange to turn the oven off and walk away, but that gentle residual heat is what transforms the bark from “crisp edges” to “fully crunchy.” Trust the process — it’s worth the wait.
Snip that piping bag carefully. When you’re using a resealable bag for the drizzle, cut the tiniest piece off the corner first. You can always snip a bit more, but you can’t go backwards — and a smaller opening gives you pretty, delicate zig-zags instead of rivers of chocolate.