Preheat oven to 350℉ (180℃). Slice strawberries and set aside.
Mix butter and sugar together in a large mixing bowl until combined. Add vanilla extract, salt, baking powder, eggs, and sourdough discard and mix.
Pour in half of the milk and half of the flour. Mix and then add the remaining milk and flour. Don't pour all the flour and milk in at once or the batter will be lumpy. Mix until smooth.
Grease a 9x13 baking dish or line with parchment paper. Pour batter into the dish.
Bake for 20-25 minutes at 350℉ (180℃) until edges start to get lightly gold brown and a toothpick inserted into the center comes out clean. The cake will be flat and light; it will not rise like a typical cake.
In a large mixing bowl while cake cools, beat together softened cream cheese, powdered sugar, and vanilla extract. Scrape down the side of the bowl as needed and beat until well combined. Fold in whipped cream with a spatula.
Spread cream cheese frosting over the cake once it has completely cooled. Add sliced strawberries on top, slice, and serve! Store in the refrigerator.