Preheat oven to 375 degrees F.
Spray the ramekins with cooking spray. Brush the sides and the bottom with cocoa powder; cover the bottom of the ramekins and the sides. rotate the ramekins until cocoa powder.
Place chocolate chips in a stainless steel mixing bowl. Place bowl over a pan of about 2 cups hot water over medium low heat. Do not let water boil or come to a simmer. Don't let the water touch the bottom of the stainless steel mixing bowl.
Melt butter and the chocolate chips in a skillet over medium heat. Sprinkle in flour. Whisk until flour is incorporated into butter and mixture thickens, about 1 minute. Reduce heat to low.
Whisk in cold milk until mixture becomes smooth and thickens, 2 or 3 minutes. Remove skillet from heat. Transfer mixture to bowl with melted chocolate. Add salt, coffee, and vanilla extract. Mix together thoroughly. Add egg yolk to the chocolate mixture and mix well.
Leave bowl above the hot (not simmering) water to keep chocolate warm while you whip the egg whites.
Place 2 egg whites in a mixing bowl; add cream of tartar. Whisk until mixture begins to thicken and a drizzle from the whisk stays on the surface about 1 second before disappearing into the mix, 2 or 3 minutes. Add sugar and whisk in. Whisk in a bit more sugar about 15 seconds. Continue whisking until mixture is about as thick as shaving cream and holds soft peaks, 3 to 5 minutes.
Transfer egg whites to chocolate mixture. Mix until egg whites are thoroughly incorporated into the chocolate, 1 or 2 minutes. Gently fold into the chocolate with a spatula, lifting from the bottom and folding over. Stop mixing after the egg white disappears. Divide mixture between 4 prepared ramekins. Place ramekins on prepared baking sheet.
Bake for 13 minutes, sprinkle some powder sugar on top, serve it hot.