Scoop the vanilla ice cream into 2-inch chunks directly into the blender to ensure even blending.

Chop the semi-sweet chocolate and melt in a microwave-safe bowl in 30-second intervals, stirring between each until smooth; let it cool slightly.

Add the cocoa powder, granulated sugar, and salt to the blender with the ice cream so the dry ingredients distribute evenly.

Pour in the whole milk, melted chocolate, and vanilla extract into the blender after the solids to prevent clumping.

Blend on high speed for 60 to 90 seconds, checking halfway for a smooth, creamy texture; add milk in 2-tablespoon increments if too thick.

If the shake is too thin, add a small scoop of ice cream and blend for 20 seconds; if too thick, add a tablespoon of milk and blend briefly.

Optional: Strain the shake through a fine-mesh strainer for extra smoothness, pressing gently with the back of a spoon.

Divide the milkshake between two tall glasses and serve immediately while it is cold and thick.

Top each glass with whipped cream and garnish with chocolate shavings before serving.