Make the cupcakes: Preheat the oven to 350 degrees F. Line a 12-cup muffin pan with liners. Whisk the flour, baking powder, baking soda and salt in a medium bowl. Whisk the melted butter, sugar, sour cream, eggs and vanilla in a large bowl until smooth. Add the flour mixture and whisk until smooth.
Divide the batter among the prepared muffin cups, filling each about two-thirds of the way. Bake until the tops of the cupcakes are golden and spring back when gently pressed, 18 to 20 minutes. Let cool 5 minutes in the pan, then transfer to a rack to cool completely.
Meanwhile, make the filling: Combine the raspberries, 1 cup water and the sugar in a medium saucepan and bring to a boil, stirring occasionally. Reduce the heat to a simmer and cook until reduced by half, about 15 minutes. Whisk the cornstarch, rosewater and 1 tablespoon water in a small bowl. Add to the raspberries and continue to cook, stirring, until thickened, 2 to 3 minutes. Strain the sauce through a fine-mesh sieve into a bowl, pressing on the solids with a rubber spatula. Let cool, about 20 minutes.
Meanwhile, make the frosting: Put the sugar and egg whites in the bowl of a stand mixer; set the bowl over a saucepan of simmering water (do not let the bowl touch the water). Whisk until the mixture is warm and the sugar dissolves, 3 to 5 minutes. Transfer the bowl to the stand mixer; beat with the whisk attachment on medium-high speed until stiff glossy peaks form, 6 to 8 minutes. Switch to the paddle attachment. With the mixer on medium-high speed, beat in the butter a few pieces at a time, making sure the pieces are incorporated before adding more. (It¿s OK if the frosting looks curdled¿it will smooth out as you beat.) Once all the butter is added, continue beating until the frosting is smooth, 1 to 2 more minutes. Add 3 to 4 tablespoons of the raspberry filling and the rosewater and beat until thick and smooth.
With a paring knife, cut a 3/4-inch-deep slit across the top of each cupcake, nearly edge to edge; gently squeeze the sides of the cupcake so the slit opens. Spoon about 1 teaspoon of the raspberry filling into each cupcake. Let stand 5 minutes, then spoon the remaining filling into the cupcakes. Spread the frosting on the cupcakes. Sprinkle with coarse sugar.
Photograph by Ryan Dausch