In a small pot, whisk together egg yolks and sugar until light in color. Whisk in corn starch, cocoa powder, and salt until combined. Whisk in whole milk. Put the pot over medium heat and bring to a boil. Let boil, stirring constantly, until it's thickened slightly. Remove from heat and stir in butter and vanilla. Cool for about 10 minutes before pouring into your pie shell. Chill completely before spreading whipped cream on top. Store in fridge until ready to serve.