For the cake: Preheat the oven to 175°C. Coat a 9-by-13-inch rectangular baking pan with nonstick cooking spray. Line the bottom of the pan with parchment paper and coat again with non-stick cooking spray. Combine the flour, cornmeal, baking powder, baking soda and salt in a small bowl. Whisk together and set aside.
Beat the butter with the granulated sugar on medium-high speed with an electric mixer until smooth and fluffy, 3 minutes. Beat in the eggs, one at a time, until combined, and then beat in the vanilla. Reduce the speed to low and beat in one-third of the flour mixture until just combined. Beat in half the buttermilk and then repeat, adding the flour in two more increments and alternating with the second half of the buttermilk. Stir in the sprinkles. Transfer to the prepared pan and spread the batter evenly. Bake until golden brown and the top springs back to a light touch, 30 to 35 minutes. Cool 15 minutes in the pan.
Run a small knife around the edges of the cake to release the sides and turn the cake out onto a baking sheet. Remove the parchment paper and then flip again onto a serving platter or cutting board so the top of the cake is back on top. Cool completely before frosting.
For the frosting: Place the butter and salt in a large bowl. Beat with an electric mixer on medium speed until fluffy. Add the white chocolate and beat until fluffy. Gradually beat in the confectioners' sugar and the vanilla. Increase the speed to medium-high and beat until very smooth and fluffy.
Transfer 1/2 cup of the frosting to a pastry bag fitted with a small rosette tip. Frost the top and sides of the cake with the remaining frosting.
For the garnish: Place the blueberries in six rows of 6 berries each in the top left hand corner of the cake. Trim and halve the strawberries lengthwise and make four rows of strawberries across the cake. Pipe 13 stars between the blueberries and use the remaining frosting to pipe a border around the bottom of the cake. Brush the berries with the melted jam. Cut and serve.