Individual Chocolate Lava Cakes

Lava:

1. Put the chocolate and salt in a small bowl.
2. Bring the milk to a simmer and pour it immediately over the chocolate. Shake the bowl to submerge the chocolate and cover it with plastic wrap for 3 minutes. 
3. Uncover and whisk until the chocolate is completely melted. Add the butter and whisk until completely blended. 
4. Refrigerate until the mixture is firm enough to scoop, 2 to 3 hours.
5. Use a spoon or small scoop to shape 8 balls; place them on a wax paper lined dish and put in the freezer until needed. 

Cakes:

1. Spray eight 6-ounce custard cups or ramekins with vegetable oil spray and coat them with sugar, tapping out the excess.
2. Put the chocolate, butter, and salt in a medium heatproof bowl (preferably stainless steel) and set it set directly in a wide skillet of almost simmering water. Stir frequently until the chocolate is almost entirely melted.
3. Remove the bowl from the skillet and stir to finish melting the chocolate. 
4. Whisk the cocoa powder, flour, and 3 egg yolks into the warm chocolate. Set aside
5. In a large bowl, beat the 6 egg whites and cream of tartar (or vinegar) on medium speed until soft peaks form when the beaters are lifted. 
6. Gradually beat in the sugar on high speed until the egg whites are stiff but not dry.
7. Fold one-quarter of the egg whites into the chocolate mixture.  Scrape the mixture over the remaining egg whites and fold just until incorporated.
8. Divide the batter evenly among the cups (being careful not to deflate it by extra mixing or handling).
9. Center a ball of lava on top of the batter in each cup.  Push the lava into the batter until it is about ¼-inch below the surface of the surrounding batter (it should not touch the bottom of the cup). Spread the batter to cover the lava.
10. Cover cups with plastic wrap and refrigerate until ready to bake.
11. Preheat the oven to 400°F with a rack in the lower third.
12. Uncover and set cups on a baking sheet.  Bake 13 to 15 minutes, until the cakes are puffed and a toothpick inserted in the center meets no resistance and emerges coated with a sauce that feels slightly warm when you touch it to your lip.
13. Let cakes rest for 3 minutes.
14. Run a thin knife around the inside of each cup, pressing against the sides to avoid tearing the cake.  Cover each cup with a serving plate, invert and lift the cup off slowly. Dust cakes with a little bit of powdered sugar (if desired), and serve immediately