How to Make It
Preheat oven to 350°F. Line a 13- x 9-inch baking pan with parchment paper, leaving a 2-inch overhang on both long sides; lightly grease with cooking spray. Beat butter and 3/4 cup sugar with a stand mixer fitted with paddle attachment on medium speed until light and fluffy, about 4 minutes. Add whole eggs, 1 at a time, beating well after each addition.
Sift together cake flour, baking powder, and salt. Add flour mixture to butter mixture in 3 additions alternately with sour cream (2 additions of 1/4 cup each), beginning and ending with flour mixture, beating on low speed until just combined after each addition. Stir in vanilla. Fold in freeze-dried strawberries. Spread mixture in prepared pan, and set aside.
Beat egg whites with stand mixer fitted with the whisk attachment on medium speed until foamy, about 30 seconds. Add cream of tartar. With mixer running on medium speed, gradually add remaining 1 cup sugar, 1 tablespoon at a time, beating until all sugar is incorporated and mixture is glossy and forms stiff peaks, about 5 minutes. Spread meringue in an even layer on top of cake batter, leaving a 1/2-inch border around edges of pan.
Bake cake in preheated oven until meringue is dry and a wooden skewer inserted through meringue and cake comes out clean, 22 to 26 minutes. Let cake cool in pan 10 minutes. Using parchment as handles, carefully transfer cake to a wire rack. (Meringue will crack.) Let cool completely, about 1 hour.
Meanwhile, place raspberry jam in a medium-size microwavable bowl, and microwave on high until smooth and melted, 30 to 45 seconds, stirring every 15 seconds. Add berries, lemon juice, 4 teaspoons sumac, and 1/8 teaspoon salt; toss to coat. Let stand at room temperature at least 30 minutes or up to 1 hour, stirring occasionally.
Combine heavy cream, sour cream, vanilla, and remaining 1/8 teaspoon salt in bowl of a stand mixer fitted with the whisk attachment. With mixer running on medium speed, gradually add sugar, beating until medium peaks form, 1 to 2 minutes.
Transfer cake to a serving board, and remove parchment paper. Top with cream mixture. Just before serving, top with berry mixture. Sprinkle with remaining 1/4 teaspoon sumac.
Find freeze-dried berries in the produce section or snack aisle of your grocery store.