In a double boiler, melt the ¾ cup chocolate chips and butter stirring until smooth. Remove from the heat to cool.
Preheat the oven to 325F.
Whisk and sift the cocoa powder, flour, baking powder, and salt in a medium mixing bowl.
In a standing mixer fitted with the paddle attachment, beat the eggs, vanilla and orange zest adding the sugar slowly at medium speed until thick and pale.
Reduce the speed to low and slowly incorporate the melted chocolate and butter. Slowly add the flour mixture just to combine. Using a spatula, fold in the remaining chocolate chips.
Lightly flour the working countertop and transfer the dough. Divide into two and roll into 15-inch logs. Transfer into the prepared baking sheet and lightly flatten with your fingertips. Lightly brush the log with egg wash.
Bake for 35 minutes or just until firm at the center. Remove the pan from the oven and then let it cool for 15 minutes.
Lower the oven temperature to 300F.
Carefully transfer the logs into the large chopping board and slice the logs into ½" thickness. Arrange the sliced cookies cut side down back into the baking sheet and bake for 20 minutes or until the cookies are dry. Cool completely and store in the airtight container.