Note on the pie: if you’re using a 9” standard pie plate it will be a very full pie, as you can see from the photos. Bake it on a cookie sheet just in case, but it should not spill over. You can use up to a 10” pie plate, but do not use anything smaller than 9”. If you have a 9” deep dish pie plate, you can use that as well.
Make the crust: Preheat oven to 350°F. Grind the graham crackers in a food processor until they are a fine crumb. (You can also place them in a large gallon size ziploc bag and roll them with a rolling pin.) Place in a bowl and stir in sugar and melted butter with a fork. Press into the bottom and up the sides of a 9-10” or 9” deep dish pie plate. Bake until firm, about 10 minutes.
Make the filling: Melt the butter with chocolate in a medium sized bowl. I did this in the microwave, heating on high in 30 second increments, stirring between each, until the chocolate is melted and smooth. (You can also do it in a bowl set over a pan of simmering water.)
Once the chocolate is melted and smooth, whisk in the flour, sugar, vanilla, and salt. Whisk in eggs and egg white, one at a time, until the mixture is smooth. Stir in the chopped peanut butter cups, if using. Pour mixture into prepared crust. Bake until the top is puffed and filling is set in the center, 35-45 minutes. Let cool, serve at room temperature. Top with whipped cream or ice cream. (Note that I get pretty slices because I chill my pies a few hours before cutting. That's the secret to perfect slices!)