Preheat oven to 325.
Line a 9" square baking pan with parchment paper.
In a medium saucepan set over low heat melt the butter and chocolate, stirring frequently. When melted and smooth remove from heat and stir in the sugar.
Add the eggs one at a time making to sure incorporate each one.
Stir in the vanilla and the espresso.
Gently stir in the flour and the salt, stirring only until the streaks of flour are almost gone.
Stir in the chocolate chips.
Pour the batter into the pan and smooth the top.
Bake 30-40 minutes or until the edges are firm, the center should be almost set. Insert a toothpick into the center, remove where there are some crumbs attached.
Remove from oven and let cool.