I love buttermilk and I'm not shy about admitting it. I'm not going to flaunt it the way people wear I ♥ foie gras t-shits, but if you see me pour the last 6 oz of a carton of buttermilk into a glass and drink it, I'm not apologizing.
Step 1: Blind bake your crust. Custard pies always benefit from a blind baking; your pie crust gets a head-start on cooking and develops a toothsome, rather than gummy integrity that contrasts well with the custard filling.
Roll your pie dough out to somewhere between a 1/8 and 1/4" thickness and transfer to a pie plate. I like to roll the edge of the dough underneath itself before fluting to give the ends a little more thickness. Refrigerate the formed pie shell for at least an hour, up to overnight. Preheat oven to 400. Spread a sheet of parchment paper or aluminum foil over the chilled dough so that it hangs a couple inches off of the sides of the pan. Pour in your beans or rice or pie weights, and bake the pie for 10 minutes. This weight gives the dough the structure it needs for baking, otherwise the sides would collapse. After 10 minutes, remove the pie from the oven and carefully remove the parchment or foil and all of the weights. Prick a few holes in the bottom of the crust with a fork and return to the oven (without weights) for another 3-5 minutes, until very lightly browned. Let cool. Pie crust can be made several hours ahead or even the day before.
In another bowl, or with an electric mixer, whisk egg whites until they form soft peaks (when you pull the whisk up through the whites, they should hold some of their shape). Gently fold egg white mixture into buttermilk mixture, by hand, until just combined. Mixture will be somewhat lumpy.
Pour filling into pie shell. Bake in middle of oven until pie is the palest shade of brown and the filling is mostly firm but for a bit of jiggle in the center, 45 to 50 minutes. Depending on how acidic the pie is, it may not brown much. The slight jiggle will tell you when it's done. If edge of crust browns too quickly, cover with foil. Cool on a rack. Serve warm or chilled, with fresh berries.