Heat treat the flour in the microwave by placing the flour in a bowl and heating on high for 30 seconds to 1 minute (Note 2).
Whisk together the wet ingredients in a large bowl, including the melted butter, brown sugar, milk and vanilla extract until combined.
Add the heat-treated flour and cocoa powder and fold using a rubber spatula. Add more milk a tablespoon at a time as needed to reach your desired consistency. A thicker brownie batter is best served with a spoon, but you can make it slightly thinner to enjoy as a dip with fruit or cookies.
Stir in chocolate chips and chocolate chunks until well-distributed. Serve immediately or store in an airtight container in the fridge for up to 5 days. Brownie batter will freeze well for up to 3 months.
