Create a double boiler by placing a heat-safe bowl over a pot with about 2 inches of simmering water. Add 4 tablespoons of butter and the 5 ounces of chocolate chips to the bowl. Stir occasionally until everything is beautifully melted and smooth. Carefully remove the bowl and let the chocolate mixture cool for about 10 minutes.
While the chocolate cools, grab a separate bowl. Using a stand mixer with a whisk attachment (or a hand mixer), beat 1 cup of cold heavy whipping cream with 2 tablespoons of powdered sugar until lovely, firm peaks form. Pop this whipped cream into the refrigerator to keep it chilled.
In a large bowl, combine the softened cream cheese, the remaining 1 ½ cups of powdered sugar, cocoa powder, 1 ½ teaspoons of vanilla, and the salt. Beat with a hand mixer until it's wonderfully smooth and fluffy, about 1 minute.
Pour the slightly cooled chocolate mixture into the cream cheese mixture. Mix on a low speed for about 30 seconds until just combined – try not to overmix!
Now for the magic! Retrieve your chilled whipped cream. Gently fold half of it into the chocolate and cream cheese mixture using a large spatula. Once almost combined, add the rest of the whipped cream and continue to gently fold until no white streaks remain. This keeps the filling light and airy!
Spoon this glorious chocolate filling over your cooled cookie crust. Use an offset spatula to spread it evenly into all the corners and level the top.
Chill the bars in the refrigerator for at least 3 hours, or ideally, overnight. This step is crucial for perfect slicing!