Chop up the cold butter into small cubes.
Add the brown sugar and granulated sugar to a bowl with a paddle attachment and mix together.
Add the eggs and vanilla and mix until combined.
In a separate bowl, mix together the cake flour, all-purpose flour, baking soda, cornstarch, salt, and dark cocoa powder.
Add the espresso powder to the wet ingredients and mix.
Add the dry ingredients to the wet ingredients and mix until just combined.
Chop up the Heath Toffee Bars and milk chocolate and fold into the dough.
Chill the dough in the fridge for a bit.
Scoop the dough into 6 oz balls and bake at 415°F for 10 minutes.
Top the fresh baked cookies with additional chopped Heath Toffee and chocolate.
