Preheat oven to 375 degrees. Beat butter and sugar together in a stand mixer until fluffy. Add the eggs, one at a time. Then add the vanilla and milk and beat until combined.
In a separate bowl, whisk the flour, salt and baking soda. Add the flour mixture into the wet ingredients until combined. Fold in the chocolate chips. Divide the dough in half.
Coat a 9x13 baking pan with nonstick cooking spray. Press 1/2 of the cookie dough down into the pan. Bake for 20 minutes. Allow to cool for 15-20 minutes before turning out of the pan. Repeat this process with the other 1/2 of the cookie dough.
Preheat oven to 375 degrees. Heat butter in a medium-sized bowl in the microwave in 20 second intervals until completely melted. Immediately mix the chocolate into the melted butter and stir quickly to melt. If the chocolate does not melt completely, microwave the mixture in 20 second intervals until the mixture is smooth. Stir in the sugar, eggs and vanilla and mix well.
In a separate bowl, whisk together the flour, baking powder, salt and cocoa powder. Fold the flour mixture into the chocolate mixture and mix well.
Coat a 9x13 baking pan with nonstick cooking spray. Press the batter into the pan in an even layer. Bake for 20 minutes. Allow to cool for 15-20 minutes before turning out of the pan.