Preheat oven to 350 F/ 177 C. Line a 18 x 13 jelly roll pan with parchment and lightly spray with oil.
Melt 8 oz of dark chocolate either in a double boiler or in the microwave (50% power for 2 minutes, stir, run for another minute, stir. If there are just a few lumps, let it sit and the heat of the melted chocolate will melt the rest. Remember to not let it sit in the closed microwave after it stops running, because moisture/water makes melted chocolate seize up). Set aside.
Whip butter and sugar until fluffy. Add beer, vanilla, and eggs. Mix until well blended, then stream in melted chocolate.
Add in dry ingredients: flour, corn starch, salt, cocoa, flax seeds, and walnuts. Mix well, then add in remaining 10 oz chocolate chips.
Spread evenly into prepared baking pan. Bake for 25-30 minutes, until a toothpick comes out clean. Don't overbake!
Cool in the pan for 30-60 minutes, until it has solidified enough to be lifted out of pan by the parchment to sit on a cooling rack. Cool completely before cutting (if you can wait that long--no one will judge you if you can't). A pizza cutter works beautifully for cutting these into 48 squares. They'll keep in an airtight container, but you can better preserve their freshness by sticking them in a zip-top freezer bag and storing them in the freezer.
Notes
Make it Gluten Free! This recipe can be made gluten free by using a gluten-free 1-to-1 baking flour in place of the wheat flour, and substituting in gluten-free beer.Recipe heavily adapted from Guinness Brownies.