Preheat oven to 350°F. Grease a 9-inch pie plate or springform pan with non-stick spray.

In a microwave-safe bowl, melt the butter and 2 cups of the chocolate chips together in 30 second intervals, stirring between each, until completely smooth. Allow to cool slightly.

In a large bowl, beat the eggs, sugar, and vanilla together with a hand mixer on medium-high speed for 3 minutes until light and fluffy.

With the mixer on low, slowly stream in the slightly cooled melted chocolate mixture until fully incorporated.

Using a rubber spatula, gently fold in the flour, baking powder, and salt just until combined. Then fold in the remaining 1 cup of chocolate chips.

Pour the batter into the prepared pie plate or springform pan. Bake for 35-40 minutes, until the center is puffed up and the top is crackly. The center will still be quite jiggly.

Remove from oven and allow to cool completely at room temperature.

Once cooled, slice and serve with hot fudge sauce drizzled over the top.