Try this Chocolate Cream Pie recipe.
Replacing unsweetened chocolate with Dutch-processed cocoa powder alters the fat-to-solids ratio and requires additional butter to maintain the proper emulsion in a custard. Starch-thickened custards rely on precise heat control to fully hydrate flour molecules without scorching the dairy base. Medium heat on a 2-quart saucepan provides the correct energy transfer for starch gelatinization.
Tempering egg yolks prevents protein coagulation and scrambling when introduced to hot liquids. Whisk the yolks continuously while streaming in a small portion of the heated milk mixture. Cook the combined mixture for exactly 90 seconds after reincorporating the yolks to deactivate alpha-amylase enzymes that otherwise break down starch bonds. Chill the poured filling for at least four hours to ensure complete set before slicing.