Preheat your oven to 350 degrees and grease a 9x13 inch pan (or two 8 inch round pans). Set aside.
In a large saucepan, melt the butter and brown sugar together over low heat until completely melted and combined. Remove from heat, and beat in the eggs, vanilla extract, and bourbon.
Stir in the flour, espresso powder, salt, and chocolate chunks until the batter is smooth. Pour into your prepared pan(s). Bake for 28-30 minutes until the bars look set, and a toothpick inserted into the pan about 1 inch from the edge comes out clean. Cool completely and enjoy!