Prepare the Cake: Preheat oven to 350°F. Lightly coat a jelly-roll pan with cooking spray. Sift together flour, baking powder, and salt in a medium bowl. Set aside.

Beat granulated sugar and butter in a large bowl with an electric mixer on medium speed until light and fluffy, 4 to 5 minutes. Add eggs, 1 at a time, beating well after each addition. Beat in vanilla. Add flour mixture, one-third at a time, to butter mixture alternately with cream soda, beginning and ending with flour mixture. Beat on low speed just until smooth after each addition. Using a spatula, fold in pastel sprinkles. Spread batter in prepared pan. Gently tap pan on countertop to release any bubbles.

Bake in preheated oven until a wooden pick or cake tester inserted in center of cake comes out clean, 20 to 25 minutes. Cool completely on a wire rack, about 1 hour.

Prepare the Strawberry-Sour Cream Buttercream: Remove and discard the packet of desiccant from strawberries. (The desiccant keeps freeze-dried fruit from getting damp.) Process berries in a food processor until crushed to a fine powder, about 1 minute. Beat powdered sugar and butter in a large bowl with an electric mixer on medium speed until well blended. Add crushed strawberries, and beat until fluffy, about 4 minutes. Add sour cream, 1 tablespoon at a time, and beat until just combined after each addition. Beat on medium-high speed until creamy and spreadable, about 3 minutes. (If necessary, beat in up to 2 tablespoons of milk, 1 tablespoon at a time, until spreadable.) Spread buttercream over cake. Garnish with sprinkles. Serve immediately, or cover and refrigerate up to 3 days.