Step 1: Prepare the Pie Crust. In a large bowl, whisk together 1 1/4 cups all-purpose flour and 1/2 teaspoon salt. Cut in the cold, cubed butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 2: Add Ice Water. Gradually add 1/4 cup ice water, one tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix. If needed, add a little more ice water, 1 teaspoon at a time.
Step 3: Chill the Dough. Form the dough into a disc, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes, or up to 2 days.
Step 4: Preheat Oven and Roll Crust. Preheat your oven to 350°F (175°C). On a lightly floured surface, roll out the chilled pie dough into a 12-inch circle. Carefully transfer it to a 9-inch pie plate. Trim and crimp the edges as desired.
Step 5: Prepare the Hot Fudge Filling. In a medium bowl, whisk together the granulated sugar, 1/2 cup unsweetened cocoa powder, 1/4 cup all-purpose flour, and 1/2 teaspoon salt until well combined.
Step 6: Combine Wet Ingredients. In a separate bowl, whisk together the melted butter, eggs, and vanilla extract.
Step 7: Mix Filling. Pour the wet ingredients into the dry ingredients and stir until just combined. Gradually stir in the milk until the mixture is smooth. If using, fold in the chocolate chips.
Step 8: Bake the Pie. Pour the filling into the prepared pie crust. Place the pie on a baking sheet to catch any potential drips. Bake for 40-50 minutes, or until the edges are set and the center has a slight jiggle. Do not overbake for a gooey center.
Step 9: Cool and Serve. Remove the pie from the oven and let it cool completely on a wire rack before slicing and serving. For best results, allow it to cool for at least 2-3 hours. Serve warm or at room temperature with desired toppings.