For the cupcakes: Preheat the oven 350 degrees F. Line cupcake pans with 24 decorative paper liners.
Whisk the water and cocoa to make a smooth, lump free paste. Set aside.
Combine the sugar and oil in a mixer and beat on medium speed. Add the eggs one at a time. Beat in the sour cream and vanilla extract. Add the flour, baking soda and salt. Mix on low until just incorporated. Mix in the cocoa paste.
Spoon the batter into the prepared cupcake pans, filling almost to the top of the liners. Bake until a cake tester comes out clean, about 25 minutes. Allow to cool.
For the fluff filling: Fill a pot that is large enough for your mixing bowl to rest on top of halfway with water and bring to a simmer.
Place your mixing bowl over the simmering water and whisk the egg whites, sugar and vanilla seeds until the mixture is warm and the sugar dissolves.
Place the mixing bowl in your stand mixer and whisk on medium-high until the meringue is cool and thick.
For the ganache: Boil the cream and corn syrup. Melt the chocolate. Pour the cream over the melted chocolate and emulsify with a hand mixer. Keep warm.
To assemble: Fill a piping bag with the fluff filling and pipe a dollop of filling into each cupcake. Dip the tops in the ganache and decorate with melted white chocolate swirls or sprinkles as desired.