Preheat the oven to 350°F (175°C). Prepare a 9-inch pie plate or springform pan by coating it with non-stick spray.
In a microwave-safe bowl, melt the butter along with 2 cups of the semi-sweet chocolate chips, microwaving in 30-second intervals and stirring between each until smooth. Set aside to cool slightly.
In a large mixing bowl, beat together the eggs, granulated sugar, and vanilla extract using a hand mixer on medium-high speed for approximately 3 minutes, or until the mixture becomes light and fluffy.
With the mixer set to low speed, gradually pour in the slightly cooled chocolate mixture, blending until fully combined.
Using a rubber spatula, gently fold in the flour, baking powder, and salt, mixing just until the dry ingredients are incorporated. Then, fold in the remaining 1 cup of chocolate chips.
Transfer the batter into the greased pie plate or springform pan, spreading it evenly. Bake for 35-40 minutes, or until the top becomes crackly and the center is puffed but still slightly wobbly.
Remove from the oven and allow the pie to cool completely at room temperature.
Once cooled, slice and serve with a generous drizzle of hot fudge sauce on top.
