Raspberry-Chocolate Angel Food Cake

For cake
Heat oven to 350u0b0. Coat bottom (not sides) of a 9" x 13" baking pan with cooking spray. Beat egg whites, 1 tbsp water, cream of tartar and salt in a bowl with an electric mixer on medium-low speed until foamy. Continue to beat, adding sugar a little at a time, until batter is fluffy. Add vanilla and beat 1 minute more. Sprinkle a small amount of the flour over top of batter and fold in; repeat 8 to 10 times or until you've incorporated all the flour. Spread batter in pan, coaxing evenly into corners with a rubber spatula, and shake pan once or twice to even out surface. Bake until cake is a light golden color and surface springs back gently to the touch, 25 to 30 minutes. Run a sharp knife around edges of cake to separate it from pan. Cool on a rack 2 hours.
For filling
Heat berry preserves in a small pan over medium heat, stirring constantly, until it reaches a gentle simmer.
For frosting
Melt chocolate in a double boiler. Take bowl off heat; stir in sour cream.
For assembly
Place a cutting board over cake pan; invert cake onto board. With a 2 1/2-inch biscuit cutter or round cookie cutter, cut 12 rounds from cake. Top 6 rounds with 1 heaping tbsp of raspberry filling (you will have some left over), then place 6 remaining cake rounds on top. Spread 1/4 cup frosting over top and sides. (You should have some frosting left over.) Top with raspberries.
For serving
Serve immediately, or refrigerate and bring to room temperature 1 hour before serving.