1 pre-baked 9-inch pie crust
1 cup granulated sugar
1/4 cup unsweetened cocoa powder
1/4 cup all-purpose flour
1/4 tsp salt
2 cups whole milk
3 large egg yolks, lightly beaten
2 tablespoons unsalted butter
Whisk Dry: In a medium saucepan, whisk together the sugar, cocoa powder, all-purpose flour, and salt.
Combine Wet: In a separate bowl, lightly beat the egg yolks with the whole milk.
Mix: Gradually whisk the egg-milk mixture into the saucepan until smooth.
Cook Filling: Cook over medium heat, stirring constantly, until the mixture thickens to a pudding consistency and begins to bubble. Continue to cook for 1 minute while stirring.
Add Butter: Remove from heat and stir in the unsalted butter until melted.
Assemble Pie: Pour most of the hot chocolate filling into the pre-baked pie crust. To create the decorative top seen in the photo, set aside a small portion of the filling (about 1/4 cup). Spoon or pipe this remaining filling in dollops around the edge and in the center.
Chill & Set: Let the pie cool to room temperature, then refrigerate until fully set (about 4 hours). Once set, slice and serve.