Preheat your oven to 350°F (175°C) and prepare a 9x9-inch baking pan with nonstick spray and parchment paper.

In a medium bowl, sift together cake flour, baking powder, and salt.

In a large bowl, beat softened butter, sugar, and oil until light and fluffy. Incorporate egg whites, vanilla, and sour cream until smooth.

Gradually add dry ingredients to wet mixture, stirring until just combined.

Pour batter into the prepared pan and bake for 35–40 minutes. Cool in pan for 10 minutes before transferring to a wire rack.

Puree half the strawberries and strain the puree. Heat with sugar and vanilla, stirring until thickened.

Bloom gelatin in cold water for 5 minutes then add to warm strawberry mixture.

Whip heavy cream to stiff peaks and fold in ground freeze-dried strawberries, if using.

Fold strawberry mixture into whipped cream until fully combined.

Pour mousse over the cooled cake, cover, and refrigerate for at least 4 hours to set.

Remove from pan, slice, and garnish with fresh strawberries before serving.