Preheat your oven to 350°F (175°C). Press pie dough into a 9-inch pie plate and chill in the refrigerator.

Toast chopped walnuts in a sauté pan over medium heat for 3–5 minutes until golden and fragrant.

In a medium saucepan, bring water to a boil, remove from heat, and melt unsweetened chocolate. Add sugar, butter, and vanilla extract.

Sift together flour, baking powder, and salt in a bowl. Set aside.

Beat softened butter and sugar in a stand mixer until light and fluffy. Add egg and mix until combined.

Gradually add dry mixture and milk to creamed mixture, alternating. Stir in vanilla extract.

Pour the cake batter into the chilled pie crust, then pour the chocolate mixture on top and sprinkle with walnuts.

Bake the pie in the preheated oven for approximately 45 minutes. Check doneness with a toothpick.

Allow the pie to cool for at least 20 minutes before serving. Enjoy warm or at room temperature.