Special equipment: two muffin tins; 18 cupcake liners
Preheat the oven to 350 degrees F. Line two muffin tins with 18 cupcake liners.
Pass the cocoa powder, all-purpose and cake flours, baking soda and baking powder through a fine-mesh sieve into a large bowl. Whisk in the granulated and light brown sugars, orange zest and salt until thoroughly combined. In a medium bowl, whisk together the eggs, sour cream, water, melted butter and vanilla until smooth. Add the flour mixture to the egg mixture and whisk until combined.
Divide the batter among the cupcake liners, filling each by about two-thirds. Bake until set and a toothpick comes out clean, about 20 minutes. Transfer to a wire rack to cool.
Frost the cupcakes with the buttercream and garnish with candied orange peel.
Place the egg whites and sugar in the bowl of stand mixer. Set the bowl over a pot of gently simmering water and heat, stirring with a wooden spoon, until the sugar has dissolved and is warm to the touch (about 160 degrees F). When you touch the mixture, it should feel completely smooth and not at all gritty.
Immediately transfer the bowl to a stand mixer fitted with the whisk attachment. On high speed, whip until glossy and stiff, about 15 minutes.
Switch to the paddle attachment and lower the speed to medium. Add the butter, piece by piece, incorporating each before adding more. Lower the speed further and mix until silky and smooth.
Remove the tops and bottoms from the oranges. Cut away the skin, removing both the peel and the white part of the skin. Cut the skin into thin strips.
Place the peels in a medium saucepan, cover with cold water and bring to a boil. Cook for 1 minute. Drain and repeat the process two more times.
In a large pan over medium heat, stir the sugar and 1 cup water together until the sugar has dissolved. Bring to a boil, add the peels and reduce to heat to low. Cook, swirling occasionally (no stirring!), until the peels are very soft and almost translucent, 45 minutes to 1 hour.
Toss the peels in sanding sugar until completely covered, then transfer to a wire rack to cool.