Prepare the strawberry filling first. In a medium saucepan, combine strawberries, sugar, water, and salt. Stir over medium heat until strawberries start to break down and the mixture is bubbling—about 10–12 minutes.

Thicken the filling. In a small bowl, dissolve the cornstarch in lemon juice, then stir this slurry into the hot strawberry mixture. Continue cooking while stirring constantly until thickened, about 2–3 minutes. Remove from heat and let it cool completely. If desired, press through a sieve to remove seeds for a smoother filling.

Preheat the oven. Set your oven to 350°F (175°C). Grease and flour two round cake pans or line them with parchment paper.

Cream the butter and sugar. In a large mixing bowl, beat the softened butter with the sugar until light and fluffy, about 2–3 minutes.

Add egg whites and oil. Gradually add egg whites, one at a time, beating well after each addition. Mix in the canola oil and vanilla extract. Combine dry ingredients. In a separate bowl, whisk together the flour, cornstarch, baking powder, and salt.

Add dry and wet alternately. Add the dry ingredients to the butter mixture in three parts, alternating with milk. Begin and end with the dry ingredients, mixing just until combined do not over mix.

Divide and bake. Pour the batter evenly into the prepared pans. Smooth the tops and bake for 28–32 minutes, or until a toothpick inserted in the center comes out clean. Let cakes cool in pans for 10 minutes, then transfer to wire racks to cool completely.

Make the vanilla buttercream. In a clean bowl, beat the butter until creamy. Gradually add powdered sugar, followed by vanilla and milk or cream. Continue beating until fluffy. Adjust consistency with more sugar or cream as needed.

Assemble the cake. Level cake layers if necessary. Spread a generous layer of cooled strawberry filling over the first cake layer. Top with the second cake layer, then frost the top and sides with vanilla buttercream. Decorate as desired. Chill slightly to set if needed.