In a large bowl, combine the flour, sugar, cocoa, baking powder and salt. Add the water, butter and vanilla; beat until smooth. Divide batter between four greased oven-proof mugs or ramekins.
For topping, combine sugar, cocoa, salt and espresso powder if desired. Using a scant 1/4 cup, top each mug with cocoa mixture. Add 3 tablespoons water to each mug. Do not stir. Place mugs on a baking sheet.
Bake at 350° for 28-32 minutes or until the cake is fluffy, with some liquid boiling up around edges. Serve warm with whipped cream, a toasted marshmallows or ice cream.
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