I found this on Tasty Kitchen, and I really like it.
2 cups evaporated milk
1 tbsp. butter
1 teaspoon vanilla
3 whole egg yolks
1-1/2 c. sugar
4 tbsp. flour
4-1/2 tbsp. cocoa
1/8 tsp. salt
1 pre-baked pie crust
Combine the milk, butter, vanilla, and egg yolks together in a saucepan.
Combine the sugar, flour, cocoa, and salt together.
Sift the dry ingredients into the saucepan. Cook on medium-high heat until thick.
Pour into a pre-baked pie crust. Top with plastic wrap, leaving no air pockets. Chill several hours. Top with whipped cream and chocolate curls.
Note: If you’re using a large pie plate, you’ll have to 1.5x the recipe.