Preheat oven to 375 degrees F.
Line a 15 by 10 by 1-inch baking pan with foil, allowing 2 inches of foil to hang over ends of pan, and grease foil well (except overhang) with 1 tablespoon butter.
Melt remaining 11 tablespoons butter with unsweetened chocolate in a large metal bowl set over a pan of barely simmering water, stirring until smooth. Remove bowl from heat and whisk in sugar, espresso powder, vanilla, and salt (mixture will be grainy), then add eggs 1 at a time, whisking after each addition until batter is smooth.
Toss together flour and chocolate chips in another bowl and add to batter, stirring until just combined.
Spread batter evenly in baking pan and bake in middle of oven until top is firm and edges just begin to pull away from sides of pan, about 20 minutes (do not overbake).
Cool in pan on a rack 5 minutes, then carefully lift brownies from pan by grasping both ends of foil and transfer to rack to cool 10 minutes more. Cut into 32 squares and lift brownies off foil with a spatula.