01 - Combine milk and heavy cream in a medium saucepan over medium heat. Warm until steaming but not boiling.
02 - Add chopped dark chocolate and cocoa powder, whisking constantly until completely melted and smooth.
03 - Stir in sugar and vanilla extract, continuing to mix until sugar fully dissolves.
04 - Add mixed berries and simmer gently for 5 minutes, occasionally mashing berries with a spoon to release their juices.
05 - Remove from heat and strain through a fine sieve into a clean saucepan or pitcher, pressing firmly to extract all liquid. Discard berry solids.
06 - Return strained cocoa to low heat briefly to warm through if necessary.
07 - Pour into mugs. Top with whipped cream, extra berries, and chocolate shavings if desired. Serve immediately while hot.