Prep the pan: Spray a 9-inch pie dish with nonstick spray or grease lightly with butter. Set it aside while you make the crust. This step takes 10 seconds and guarantees the crust won't stick.
Make the crust: In a medium bowl, whisk together the graham cracker crumbs, cocoa powder, and light brown sugar until evenly combined — the brown sugar should be broken up, no clumps. The cocoa will smell deeply chocolatey. Stir in the melted butter with a fork until every crumb is moistened; the mixture should clump together when squeezed. Press the mixture firmly and evenly into the bottom and up the sides of the prepared dish. I use the bottom of a measuring cup to really pack it down — spend a full 2 minutes on this. Chill the crust in the freezer while you make the filling — at least 15 minutes, but it can hang out longer if you need.
Whip the cream: Using a hand mixer or a stand mixer fitted with the whisk attachment, beat the heavy cream and ¼ cup of the powdered sugar on medium-high speed until stiff peaks form — about 4-5 minutes. You'll hear the sound change as the cream thickens, from a slosh to a soft whoosh. When you lift the whisk, the cream should hold a peak that doesn't droop. Spoon the whipped cream into a small bowl and set it aside in the fridge. Do NOT wash the mixing bowl — you'll use it next.
Make the peanut butter base: Using the same mixing bowl (no need to clean it!), beat the cream cheese, peanut butter, vanilla, and remaining ¾ cup powdered sugar together on medium-high speed until well combined and completely smooth. Stop and scrape down the sides with a spatula once or twice. The mixture will be thick and look almost like frosting. This step takes about 2-3 minutes — don't rush it, because lumps of cream cheese are not pleasant. The aroma of peanut butter and vanilla will fill your kitchen.
Fold it together: Fold in the crushed Butterfinger pieces and the reserved whipped cream until smooth and combined. Use a gentle folding motion — start from the bottom, lift, and turn the bowl. Don't stir; you don't want to deflate all that air you just whipped into the cream. The mixture will be light, fluffy, and full of crunchy bits. You'll see tiny flecks of Butterfinger throughout.
Fill and chill: Pour the filling into the prepared pie shell and spread it evenly with a spatula. Smooth the top. Freeze for 3 hours or chill in the fridge at least 5 hours or overnight. I usually do freezer if I'm in a hurry — the texture is firmer, almost like an icebox pie. Fridge gives you a softer, mousse-like texture. The pie will look glossy and set.
Top and serve: Once thoroughly chilled, top with the remaining Butterfinger pieces and drizzle with melted peanut butter (optional plain or mixed with cocoa powder). The drizzle looks fancy and adds another layer of flavor, but it's completely optional. Serve frozen (slice with a warm knife) or refrigerated. When you cut, listen for the slight crackle of the candy topping.
Store leftovers: Store in the fridge, covered, for up to 3 days, or in the freezer for up to 1 month. If frozen, let it sit at room temperature for 10 minutes before slicing.