Preheat the oven to 350°F (175°C). Stir the chocolate wafer cookie crumbs with 6 tablespoons melted butter until evenly moistened, then press firmly into the bottom and up the sides of a 9-inch pie pan to form a crust.
Bake the crust for 8 minutes, then remove and let cool completely on a wire rack while keeping the oven on.
Make the cookie dough: in a large bowl, beat 1/2 cup room-temperature butter with 3/8 cup granulated sugar and 3/8 cup packed brown sugar on medium speed until light and fluffy. Add 2 1/2 tablespoons milk and 1/2 teaspoon vanilla and mix to combine.
Reduce speed to low and add 1 1/4 cups all-purpose flour, 1/4 teaspoon baking soda, and 1/2 teaspoon salt; mix until just incorporated. Stir in 1/2 cup mini semi-sweet chocolate chips.
Form about eight 1/2-inch dough balls from some of the dough and place on a parchment-lined baking sheet; bake 7–9 minutes until edges are lightly golden. Cool on the sheet a few minutes, then transfer to a wire rack to cool completely. Reserve the remaining dough for the pie.
For the filling, whisk 3/4 cup light brown sugar, 1/3 cup all-purpose flour, and 1/4 teaspoon salt in a medium saucepan. Stir in 1 cup of the milk until smooth, then bring to a boil over medium heat, stirring constantly, until thickened (about 2 minutes). Remove from heat to cool slightly.
Beat the 3 egg yolks with the remaining 1 cup milk in a bowl. Temper the yolk mixture by slowly whisking in a small amount of the slightly cooled milk mixture, then pour the tempered yolks back into the saucepan and return to medium heat.
Bring the combined mixture to a gentle boil again, stirring constantly, then reduce heat and simmer about 1 minute until pudding-thick. Remove from heat and stir in 1 tablespoon unsalted butter and 1 teaspoon vanilla. Set aside to cool slightly.
While the filling cools, break off pieces of the reserved cookie dough and press evenly into the cooled crust to form a layer about 1/2 inch thick.
Pour the warm (not piping hot) filling over the cookie dough layer, leveling the surface; the filling should come up about 1/2 inch from the top of the crust. Refrigerate until completely chilled and set.
Whip 1 cup heavy cream until soft peaks form, then add 3 tablespoons sugar and 1/2 teaspoon vanilla and continue beating to stiff peaks. Spread all but 1/3 cup of the whipped cream over the chilled filling, smoothing to a slight dome.
Use the reserved 1/3 cup whipped cream to pipe or dollop small mounds around the edge of the pie and garnish as desired.