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

Sift together the cake flour, baking powder, and salt in a medium bowl.

Cream softened butter, sugar, and oil in a large mixing bowl until light and fluffy.

Add egg whites, vanilla extract, and sour cream to the butter mixture, mix until combined.

Gradually add dry ingredients to the wet mixture, mix just until incorporated.

Pour the batter into the pan and bake for 35-40 minutes, cool for 10 minutes before transferring to wire rack.

Puree strawberries in a blender and strain to remove seeds, combine with sugar and vanilla, simmer for 5-7 minutes.

Bloom gelatin in cold water for 5 minutes, then stir into warm strawberry puree until dissolved and cool for 30 minutes.

Whip heavy cream in a chilled bowl until stiff peaks form.

Fold the cooled strawberry mixture into the whipped cream, add freeze-dried strawberries if using.

Pour the mousse over the cooled cake and spread evenly, chill for at least 4 hours.

Garnish with fresh strawberries, slice and serve chilled.