Place chocolate chips and butter in large microwave-safe bowl. Microwave for 30 seconds, stir, and microwave for another 30 seconds. Repeat until melted. Set aside to cool down.
Whisk yolks and cocoa powder together until combined. Add water and milk and whisk until combined. Set aside.
In a bowl fitted for a stand mixer, add egg yolks, cream of tartar, and salt together. Beat on medium high until mixture begins to foam. Gradually add in sugar while mixer is on medium low. Raise speed to medium high and beat until stiff peaks form.
Add yolk mixture to whites mixture. Fold together gently. There may be some leftover liquid that won't fold in. Fold half of egg mixture into warm chocolate mixture. Add the remaining egg mixture and fold into chocolate mixture until combined. Be careful not to deflate the egg whites. Cover with plastic wrap so it's touching the batter. Refrigerate until pastry cream is ready.
Preheat oven to 350 degrees. Prepare ramekins by greasing and then coating in sugar. Scoop chocolate cake mixture with a medium ice cream scoop and place a scoop into each ramekin. With a small cookie scoop (about a tablespoon), scoop peanut butter pastry cream on top of each chocolate scoop and push into chocolate. Place another chocolate scoop on top and spread into an even layer.
Bake souffles in preheat oven for 23-25 minutes. Do not open oven until souffles are at least 22 minutes, or else they will fall. Remove from oven when done and serve immediately.