Preheat your oven to 350 degrees Fahrenheit (180 degrees Celsius).
Prepare your fresh plums. Rinse them well and cut them in half to expose the seed core. Take out and discard the seed core. Depending on the size of each plum, cut them into quarters or halves. Collect them in a bowl and keep them aside.
1 Pound Plums
In a mixing bowl, combine first the dry ingredients (without the sugar), the flour, the salt, the baking powder, and the vanilla sugar. Keep aside.
In a larger bowl, blend the soft butter and the sugar until you have a pastry mix.
1¼ Stick Butter, ¾ Cup Sugar
Add the eggs to the butter-sugar mix and blend them in until all is smooth.
3 Eggs
Pour and combine the milk into the egg, butter, and sugar pastry mix.
¼ Cup Milk
Gradually add the previously prepared combination of flour and dried ingredients to the egg buttermilk batter and beat it all well until the batter appears light yellow/beige and until it's smooth in texture.
Line your baking sheet with parchment paper and pour the batter over that. Spread the batter evenly over the parchment paper.
Place and arrange the plums over the raw batter.
Bake the slab plum cake at 350 degrees Fahrenheit (180 degrees Celsius) for about 25 to 30 minutes or until golden and cooked through. You can test if the cake is done baking by poking into the cake with a pointed knife at the end of the baking time. If it comes back clean, your cake is ready.
Notes
The plum quantity is just a base recommendation. I used about a pound of plums, but you can add a lot more if you want a juicier cake. In fact, you can fill up the top with plums if you have a surplus of fruits.