Chocolate Raspberry Angel Food Cake

Preheat the oven to 350u0b0F with the rack in the middle. Line the bottom of the springform pan with a round of parchment paper, anchoring it to the pan with a tiny smear of butter in the center.
Sift 1/2 cup sugar with the cocoa, the cake flour, and the salt onto a sheet of wax paper.
In a large bowl with an electric mixer at medium-high speed, beat the egg whites with the cream of tartar until the foam just holds a soft shape. Gradually add the remaining 1/2 cup sugar, beating, along with the vanilla, and continue to beat just until the meringue barely holds soft peaks (keep it loose; don't go all the way to stiff peaks).
Sift the cocoa mixture over the meringue and with a large whisk fold it into the meringue gently but thoroughly.
Gently transfer the batter with a rubber spatula to the ungreased springform pan, spreading it evenly and smoothing the top, and scatter the raspberries over the top. (The batter will almost fill the pan. For safekeeping put a sheet of foil on the bottom of the oven to catch any overflow from the cake pan, but I made the cake twice and it never spilled over.)
Bake the cake until a wooden skewer inserted in the center comes out clean, about 40 minutes (the cake will rise above the pan, but it doesn't spill over). Let the cake cool completely in the pan on a rack.
Run a thin knife around the edge to free the cake from the side of the pan before removing the side of the springform pan. Dust the top of the cake with confectioners sugar, if desired.