Preheat oven to 350°F (180°C) and line two 8" or 9" cake pans with parchment and grease the sides.
In a large bowl, whisk flour, baking powder, and salt together.
In a separate large bowl, beat butter and sugar together until light and fluffy, about 3 minutes. Add oil vanilla extract, and combine.
Measure out milk in a liquid measuring cup. Add egg whites and whisk to combine.
Alternate adding milk mixture and dry ingredients into the cake batter, starting and ending with the milk mixture. Mix until just combined after each addition.
Divide cake batter evenly between pans. Bake for 35-40 minutes, or until a toothpick inserted in the center of the cakes comes out clean. Let cake cool in their pans for 5 minutes, and then carefully turn them out onto a wire rack, to cool completely.
In a medium saucepan over medium heat, combine water and cornstarch. Stir occasionally until mixture thickens and is smooth.
Add sugar and strawberries and stir well. Cook the mixture over medium heat until the strawberries soften, about 3 minutes. Mash the berries with a potato masher or a fork.
Continue cooking and stirring until the filling thickens, about 5 minutes. Add lemon juice and salt and stir to combine. Let filling cool at room temperature, and then store in the refrigerator until it cools, at least 2 hours or up to 5 days.
Beat butter in a large bowl until fluffy, about 3 minutes. Add powdered sugar and beat until combined, about 3 minutes.
Beat in vanilla extract and heavy whipping cream until the buttercream is smooth.
Cut domes off the cake layers, if necessary. Place one cake layer on a serving plate. Pipe a ridge around the top of the cake with buttercream frosting, to keep the strawberry filling from oozing out of the cake. I used an Ateco 808 piping tip.
Spoon half of strawberry filling on top of the cake, within the buttercream border. Place the second cake layer on top, and frost the whole cake with buttercream, reserving a cup for decoration.
Pipe a border around the top of cake. I used an Ateco 847 piping tip. Fill the top of cake with remaining strawberry sauce.
Serve immediately after assembly. Store leftovers in the refrigerator for up to 5 days.
Don't overmix the cake batter. The dry and wet ingredients should be mixed together until just combined.
Divide the batter evenly between the two cake pans so the layers are even. I use a kitchen scale.
You can use fresh or frozen strawberries. There is no need to thaw frozen berries before using.
If using frozen strawberries, use sliced strawberries so they will cook faster.
If you want the strawberry filling smoother, puree it after it cools.