Step 1Heat the oven to 350 degrees. Line an 8-inch-square baking dish with aluminum foil, leaving a 1-inch overhang. Lightly butter the bottom and sides of the foil.
Step 2Melt the butter and bittersweet chocolate in a medium metal bowl set over simmering water until the mixture is smooth. Stir in the brown sugar until dissolved. Remove from the heat and stir in the vanilla and Grand Marnier, then the grated orange peel.
Step 3Whisk in the eggs, one at a time, until completely blended and the mixture is shiny and smooth; the mixture will be thickened at this point.
Step 4Sift together the cocoa, flour and salt, then stir the flour mixture into the batter until blended. Fold in the mini chocolate chips.
Step 5Spoon the batter into the prepared pan, smoothing the top. Bake for 35 to 40 minutes, until puffed and almost set. Do not overbake. Cool while still in the pan, on a wire rack.
Step 6Lift the brownies out of the pan, using the foil as a handle, and invert the brownies onto the cooling rack. Gently peel off the foil, then return the brownies to their original position. Cool completely. Cut into 25 squares, trimming off the edges if desired.