Prepare the pan: Line an 8x8-inch square pan with parchment, leaving overhang for easy lifting. Lightly grease the sides.
Make the crust: Mix cookie crumbs, melted butter, sugar, and a pinch of salt until the texture resembles wet sand.
Press firmly into the pan. Chill while you make the filling.
Bloom the gelatin: Sprinkle gelatin over 1/4 cup cold water in a small bowl. Let stand 5 minutes until spongy.
Whip the base: In a large bowl, beat cream cheese and sugar until smooth and fluffy.
Blend in sour cream, cocoa, vanilla, vinegar, and salt.
Add the color and tang: Mix in buttermilk, then add red gel food coloring a little at a time until you reach a deep, vibrant red. Go slow; gel is potent.
Melt the gelatin: Gently warm the bloomed gelatin in the microwave in 5–10 second bursts or over a hot water bath until just dissolved. Do not boil.
Stabilize the filling: With the mixer running on low, stream the warm gelatin into the red velvet mixture until fully combined.
Assemble: Pour the filling over the chilled crust. Smooth the top.
Tap the pan to release air bubbles.
Chill: Refrigerate at least 4 hours, preferably overnight, until set and sliceable.
Make the topping: Whip cold heavy cream, powdered sugar, and vanilla to medium peaks. Spread over the chilled red velvet layer.
Garnish and serve: Add chocolate shavings or a sprinkle of crumbs. Lift out using the parchment and cut into squares with a warm, dry knife.