1½ cups all-purpose flour
¾ cup unsweetened Dutch-process cocoa powder
1½ cups sugar
1½ teaspoons baking soda
¾ teaspoon baking powder
¾ teaspoon salt
2 large eggs
¾ cup buttermilk
3 tablespoons vegetable oil
1 teaspoon pure vanilla extract
¾ cup warm water
Salted Caramel Filling
Fleur de sel, for garnish
Dark Chocolate Frosting
Preheat oven to 350ºF. Line mini muffin tins with paper liners. In a mixing bowl, whisk together flour, cocoa, sugar, baking soda, baking powder, and salt. Add eggs, buttermilk, oil, vanilla, and the water. With an electric mixer on low speed, beat until smooth and combined, scraping down sides of bowl as needed.
Divide batter evenly among lined cups, filling each about two-thirds full. Bake, rotating tins halfway through, until a cake tester inserted in center comes out clean, about 15 minutes. Transfer tins to wire racks to cool 10 minutes; turn out cupcakes onto racks and let cool completely. Cupcakes can be stored overnight at room temperature, or frozen up to 1 month, in airtight containers.
To finish, use a paring knife to cut a cone-shaped piece (about ½ inch deep) from the center of each cupcake (discard pieces, or drizzle them with some salted caramel and ENJOY!) Spoon 1 to 2 teaspoons warm filling into each hollowed-out cupcake. Sprinkle a pinch of sea salt over filling.
Fill a pastry bag fitted with a medium open star tip with frosting. Pipe frosting onto each cupcake, swirling tip and releasing as you pull up to form a peak. Garnish each cupcake with a pinch of sea salt. Cupcakes are best eaten the day they are filled and frosted; store at room temperature in airtight containers (do not refrigerate) until ready to serve.