Preheat the oven to 350°F / 175°C. Butter a 7" springform pan, then coat the inside with sugar.
To a medium sized mixing bowl, add the flour, baking powder, salt, sugar, and cubed butter. Use a hand mixer or a stand mixer fitted with the paddle attachment to beat the mixture together until the butter has completely combined with the dry ingredients and no lumps remain.
Add in the oil, vanilla, sour cream and milk. Whisk the ingredients together just until a cohesive batter has formed.
In a separate bowl, beat the egg whites until they reach stiff peaks.
Add the egg whites to the batter and gently fold them into the batter until the mixture is cohesive.
Pour the cake batter into your prepared pan. Bake for 35-45 minutes, until a toothpick inserted in the center of the cake comes out with only a few moist crumbs.
While the cake is baking, make the strawberry jam.
Strawberry Jam
In a medium sized saucepan set over medium heat, mix together the strawberries, sugar, and lemon zest. Stir frequently, mashing the strawberries down as much as you can as they soften.
In a small bowl, whisk together the cornstarch and lemon juice. Add it to the jam, and stir continuously until the mixture thickens.
Remove the jam from the heat and set it aside until you're ready to assemble the cake.
Vanilla Pudding
Add the instant vanilla pudding mix to a medium sized mixing bowl. Add in the heavy cream and whole milk and whisk for 2 minutes.
Transfer the pudding to the fridge to chill until you're ready to assemble the cake.
Once the cake has completely cooled to room temperature, slice it in half horizontally to get two even pieces. If the cake seems a little over-baked/dry, brush some milk onto the inside part of both halves.
Place the bottom half of the cake into the pan. Place the halved strawberries standing up and facing outwards towards the cake pan, along the outer ring (I have pictures of this earlier in the post for visual reference.) You'll want to make sure the strawberries are short enough that they don't peak out of the top of the pan, so that the top cake layer can sit flat on top of the pudding/strawberry mixture.
Add ½ of the pudding mixture into the center of the cake and carefully spread it into an even layer, making sure it spreads into the crevices between the strawberries. Spread the jam into an even layer over the pudding, then add the remaining pudding on top. Use a knife to swirl the jam into the pudding.
Place the top half of the cake on to the pudding - pushing down slightly to secure it.
Cover the cake and place it in the fridge while you make the whipped cream.
Whipped Cream
Add the heavy cream, sugar, vanilla and salt to a medium sized mixing bowl (or to the bowl of a stand mixer fitted with the whisk attachment.) Beat the mixture until soft peaks form.
Remove the cake from the fridge and from the pan. Top it with the whipped cream and extra strawberries. Slice, serve, and enjoy!