Have all the ingredients at room temperature.
Preheat an oven to 325°F. Generously grease and flour an 8-well cream boat pan.
To make the cakelets, over a sheet of waxed paper, sift together the flour, baking soda and salt; set aside.
In a small bowl, whisk together the butter, egg and vanilla; set aside.
In a large bowl, whisk together the sugar and cocoa powder. Whisk in the boiling water, then whisk in the butter mixture. Add the flour mixture in two additions, whisking until smooth and no lumps of flour remain.
Divide the batter among the wells of the prepared pan. Bake until the cakelets spring back when touched and a toothpick inserted into the center comes out clean, 17 to 20 minutes. Transfer the pan to a wire rack and let the cakelets cool for 10 minutes. Invert the pan onto the rack, gently tap the pan bottom and lift off the pan. Let the cakelets cool completely.
Meanwhile, make the filling: In the bowl of an electric mixer, using a handheld whisk, beat together the egg whites, sugar, corn syrup and salt just until combined. Place the bowl over but not touching simmering water in a saucepan and whisk until the sugar dissolves and the mixture is hot, 2 to 3 minutes.
Set the bowl on the mixer fitted with the whisk attachment and beat on medium speed for 3 minutes. Add the vanilla and beat until the outside of the bowl is cool and medium-firm peaks form, 2 to 3 minutes more.
Spoon the filling into a pastry bag fitted with a 1/4-inch plain tip. Insert the tip 1 inch into the bottom of a cakelet. Gently squeeze the filling into the cakelet while slowly withdrawing the tip. Repeat in two more places along the bottom. Turn the cakelet over so that the filling holes are on the underside and place on a serving plate. Repeat with the remaining cakelets and filling. Makes 8 cakelets.