Combine flour, baking soda, espresso powder and salt in a small bowl and whisk well.
In a medium-sized bowl, cream together softened butter and brown sugar with a handheld mixer for several minutes until the butter and sugar mixture has turned lighter in colour and is smooth and fluffy looking.
Incorporate one room temperature egg, one egg yolk and vanilla extract.
Add the dry ingredients to the wet ingredients and stir until just combined.
Roughly chop 5 oz of semi sweet bakers chocolate. Set aside approximately 1 oz of the chocolate to use to top the cookie dough balls. Stir in 4 oz of the baker's chocolate to the dough and set the dough in the fridge for a minimum of 1 hour or till next day.
When you’re ready to bake, preheat the oven to 375 degrees F. Prepare two cookie sheets with parchment.
Roll cookie dough into balls using approximately 1 ½ tablespoon of dough. Top the dough with 3-4 small pieces of your reserved chopped chocolate.
Bake for 8-10 minutes. (If you rolled your cookies into slightly larger balls, they will need to bake 10-12 minutes). When the cookies look almost set in the middle, remove them from the oven. Sprinkle with flaky salt if desired.