Heat your oven to 350F.
In a mixing bowl, combine the sugars and eggs. In a separate bowl, combine flour, cocoa powder, and salt.
In a small saucepan, combine the butter, milk, whiskey, and espresso (if using). Bring to a simmer, then stir in the chocolate chips, until melted. Remove from the heat and let cool for a minute or two.
Stir the chocolate mixture into sugar and eggs. Fold in the dry ingredients.
Pout the batter into a 9x9 baking pan and bake for 30 minutes, or until the center is set.
Let cool before slicing.