First, preheat the oven to 350F, and line two muffin pans with 28 cupcake liners (you might get slightly less than 28!)
Whisk the flour, cocoa powder, baking powder, baking soda and salt together in a large bowl.
In a separate bowl, whisk together the eggs. Add the granulated sugar, brown sugar, oil and vanilla and whisk until combined.
Add half of the wet ingredients to the dry ingredients, and stir lightly to combine. Add half of the buttermilk and stir again. Repeat, finishing with the buttermilk.
Scoop the batter into the cupcake liners and fill them only 1/2-way full.
Bake the cupcakes for 18 minutes. Open the oven, test with a toothpick to see if they’re done. If not, return to the oven for 2 minute increments until done.
Remove from the oven, and let cool.
While the cupcakes cool, brown the butter: melt it in a light-colored skillet (it’s hard to monitor the ‘browness’ of the butter if the pan is dark). Let the butter sizzle over medium-low heat—it will sputter at first (this is the water leaving the butter), and then pay close attention as the sputtering stops. Tilt the pan to gently swirl it, and monitor the formation of brown bits on the bottom. Remove the pan from the heat when the butter liquids are a light golden brown and brown bits are forming. Pour into a bowl, and refrigerate for one hour to semi-solidify.
When the cupcakes are cool, make the buttercream! Place the butter in the bottom of a stand mixer fitted with the whisk attachment (discarding any black bits but adding the brown bits!). Turn the mixer to medium and whip the butter. Slowly begin to add the powdered sugar. About half-way through, splash in some cream to get it going. Add the rest of the powdered sugar and cream. After beating for a few minutes, stir in the vanilla and salt. Give it a taste and add more salt to make the flavors pop—it should really taste amazing. If it tastes flat, it needs more salt, trust me on this! Also, add additional cream to make the buttercream soft and fluffy.
You can decorate the cupcakes simply with a knife, or scrape it into pastry bag fitted with a pretty 1M Wilton tip. Pipe each cupcake with a rose shape (start in center and make a circle will gently pulling up). Decorate with sprinkles and serve.