Heat oven to 335º F/168º C — 350º F/177º C. I tend to use lower setting to prevent my cakes from getting too dark on the outside before the inside is done baking. Make sure all ingredients are at room temperature or slightly warm (eggs, buttermilk, butter)
Measure out buttermilk. Place 4 oz in separate measuring cup. Add oil to the 4oz of buttermilk and set aside. To the remaining milk, add your eggs (lightly whisked to break them up) extract, lemon juice and lemon zest.
Measure out dry ingredients and place them into the stand mixer bowl.
Attach the paddle to the mixer, and turn on the slowest speed (setting 1 on Kitchen Aid mixers). Slowly add chunks of your softened butter until it is all added. Let mix until batter resembles coarse sand.
Add your milk/oil mixture all at once to the dry ingredients and mix on medium (speed 4 on kitchenaid) for 2 full minutes.
Scrape the bowl. This is an important step. If you skip it, you will have hard lumps of flour and unmixed ingredients in your batter. If you do it later, they will not mix in fully.
Slowly add in the rest of your liquid ingredients in 3 parts, stopping to scrape the bowl one more time halfway through. Your batter should be thick and not too runny.
Lightly grease 2 8" cake pans cake goop or other pan release. Fill pans 3/4 full. Give the pan a little tap on each side to level out the batter and get rid of any air bubbles.
Add 3-4 large dollops of your raspberry filling to the cake batter and use a spoon or knife to swirl it through the batter. I also add in about 1/4 cup of fresh raspberries dusted with flour sprinkled on top. Do not stir them in.
I always start by baking for 30 minutes for 8" and smaller cakes and 35 minutes for 9" and larger cakes and then checking for doneness. If the cakes are still really jiggly, I add another 5 minutes. I check every 2 minutes after that until I'm close and then it's every 1 minute. Cakes are done when a toothpick inserted in the center comes out with a few crumbs.
After cakes have cooled for 10 minutes or the pans are cool enough to touch, flip the cakes over and remove from the pans onto the cooling racks to cool completely. Wrap in plastic wrap and chill in the refrigerator.
Once the cakes are chilled in the refrigerator and feel firm you can trim off the brown edges from the sides (optional) and fill with your buttercream and more of your raspberry filling. It's best to create a dam (see video) to help hold in the raspberry filling and keep it from oozing out. Crumb coat the cake with buttercream, chill then apply your final coat of buttercream. Top with lemon slices, more raspberries and a drizzle of raspberry filling around the outside edge.