Preheat oven to 350 degrees F. Grease an 8 inch square cake pan and line with parchment paper.
In the bowl of your electric mixer, add the flour, sugar, baking powder, and salt. Mix for 20 seconds on low.
Next, add the butter, vanilla, milk, and buttermilk. Mix on medium speed until the ingredients are just combined. Make sure to scrape down the sides of the bowl with a spatula to ensure that all the ingredients are well mixed.
In a small bowl, mix the egg and oil together with a whisk. With your mixer on low speed, slowly add the egg and oil mixture to the batter. Mix for 15 seconds and then stop.
Pour your batter into your prepared cake pan.
Spread your strawberry mixture evenly over the top of your batter.
Add your crumble to the top of the cake and strawberries, making sure that it is evenly distributed over the desert.
Bake for 45-60 minutes, or until the cake is done. If you are unsure, if your cake is fully cooked, insert a clean toothpick into the center of the cake. If it comes out clean, it's done. Set your cake on a wire cooling rack and allow it to completely cool.