Preheat the oven to 350ºF (175ºC). Grease and flour three 8-inch⋇ cake pans, set aside.
Sift or stir together the flour, baking powder, baking soda, and salt. Set aside.
Using a mixer, beat the butter on medium speed until smooth. Add the sugars and the raspberry jam**. Turn speed up to medium high, continue mixing until the butter mixture is fluffy and lighter in color, about 3-5 minutes. Scrape down the bowl.
On medium-low speed, add the vanilla, then the eggs one at a time, then the egg yolk, scraping down the bowl after each addition.
Alternate addition of the flour and buttermilk on low speed in three stages, beginning and ending with the flour. Once dry ingredients are just incorporated, turn mixer speed up to medium and mix no more than 30 seconds.
Evenly divide the batter into the prepared pans and bake about 20-25 minutes, until a toothpick comes out clean and the sides of the cake begin to pull away from the pan.
Let cool in the pan on a metal rack for at least 10-15 minutes, then turn the cake out onto the cooling rack to finish cooling.