Melt 1 stick of butter with dry milk powder in a medium saucepan over medium heat, stirring often and scraping bottom of pan with a heatproof rubber spatula, until butter foams, then browns, about 4 minutes. Transfer butter to a large heatproof bowl and let cool 1 minute. Cut remaining ½ stick butter into small pieces and add to brown butter and stir until melted.
Whisk in dark brown sugar, granulated sugar, egg, egg yolks, and vanilla and stir until smooth.
Combine flour, baking soda, and salt - stir, then add to the wet ingredients and stir until combined.
Fold in chocolate chips and toffee bits.
This next step is completely unnecessary but I found this gave me the most uniform cookies. I call it the Toll House method.
Line an 8x8 baking dish with parchment paper and add all the cookie dough leveling the dough with a rubber spatula until the top is as even as possible. Then refrigerate for about 20 minutes.
During this time, Preheat your oven to 375°F.
Once your dough is set, take it out of the dish and cut into 36 even squares. I cut once down the middle then cut each half into three then turn 90° and repeat.
Roll each square into a ball and baking on an ungreased cookie sheet for 8-10 minutes.