Preheat oven to 375F/190C and line two cookie sheets with parchment paper.
In large, microwave safe bowl, combine butter and 1 cup (170) of chocolate chips. Microwave for 30 seconds and stir well. Return to microwave and heat for 15 seconds at a time (stirring after every 15 seconds) until chocolate and butter are completely melted and combined.
Stir in cocoa powder and then sugar (mixture may be thick, that's OK).
Add egg whites and vanilla extract, stir well.
In separate, medium-sized bowl, whisk together flour, baking soda, and salt.
Gradually stir flour mixture into chocolate mixture until completely combined.
Stir in milk and water until evenly combined.
Divide the batter between the two prepared cookie sheets, pouring half onto one sheet and half onto the other.
Use a spatula to spread the batter very thinly (think the thickness of a potato chip) and evenly (try to keep the edges even, otherwise they will be more likely to burn). Sprinkle each half with 2 Tablespoons (22g) of the remaining chocolate chips.
Bake (it's best to bake the sheets one at a time) for 15-17 minutes. Keep a close eye on the brownie brittle so that the edges don't burn. If your oven does not bake evenly, I recommend turning the pan halfway through baking.
Remove from oven and immediately use a large knife or pizza cutter to cut shapes into the brownie brittle.
Allow the brittle to sit, undisturbed, until cool.
Break brittle along the cut lines that you made with knife/pizza cutter.
Set pieces on wax paper or cooling rack and melt your dark chocolate chips or melting wafers at 15-second increments (stirring between each) until completely melted.
Use a spoon or knife to drizzle chocolate over brownie brittle pieces (you may also transfer the melted chocolate to a small ziploc bag and snip a small hole in one of the corners and drizzle the chocolate that way).
Allow the chocolate to set and harden before serving and enjoying.