In a medium saucepan, add the raspberries, granulated sugar, lemon juice, and lemon zest. Set the saucepan over medium heat and stir with a silicone spatula to combine. Cook until the mixture begins to boil, mashing down the raspberries as they soften.

Once the mixture starts to boil, lower the heat and simmer for 10-15 minutes while you continue stirring occasionally. Then, remove from heat and use a fine mesh sieve to strain out the seeds over a medium bowl. Use the silicone spatula to press the raspberry mixture into the sieve, scraping the other side of the sieve often to capture the strained raspberry mixture. After straining, you’ll be left with about ¼ Cup of seedy pulp.

In a small separate bowl, combine the cornstarch and water until uniform. Return the strained raspberry filling to the saucepan (wipe it free of any remaining seeds if needed) and add the cornstarch/water mixture. Set the saucepan over medium heat and bring the mixture back to a boil, stirring constantly. Once it's boiling, reduce the heat to a simmer and let it cook for 1-3 minutes, until the filling has thickened slightly, is no longer cloudy, and coats the back of the spatula.

Pour the raspberry filling into an airtight container and refrigerate for at least 4 hours and up to 1 week. It will be jello-like once it sets but after stirring it will be more of a thick, jammy consistency.

Preheat the oven to 350°F (177°C) and prepare three 6-inch or two 8-inch cake pans by spraying the sides with baking spray and fitting a parchment paper circle to the bottom of each pan.

In a medium bowl, add the flour, baking powder, baking soda and salt. Whisk together and set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter at high speed until creamy, about 2 minutes. Add the sugar and cream it together with the butter at medium-high speed until light and fluffy, about 2 minutes, scraping the bowl and paddle at the halfway point. Turn the mixer to low and add the egg whites one at a time, mixing until they are just combined and scraping down the bowl and paddle as needed. Add the vegetable oil and vanilla, increase the mixer speed to high and beat for 1 minute.

Turn the mixer off and add the flour mixture all at once. Mix the ingredients on low speed until just combined, then slowly pour in the buttermilk. Continue mixing at low speed for about 30 seconds, until the batter is uniform and combined. The batter will be slightly thin.

Divide the batter evenly between the prepared cake pans and bake for 28 to 32 minutes. They’re done when they spring back to the touch and a toothpick inserted into the centers comes out clean or with just a few moist crumbs on it. Let the cakes cool in their pans for 5 minutes, then remove them from the pans and allow them to cool completely on a wire rack.

With a food processor, grind the raspberries into a fine powder. Sift out the seeds (if a few seeds end up in the powder that is totally ok) and set the powder aside.

In the bowl of a stand mixer fitted with the paddle attachment (or with a hand mixer and large bowl), beat the butter on medium-high speed until pale and creamy, about 5 minutes. Add the raspberry powder and milk and continue to mix on medium until fully combined.

Turn the mixer to low speed and add the powdered sugar a few cups at a time, scraping down the bowl and paddle after each addition. Add a pinch of salt and continue mixing on low speed until fully combined and smooth, 1-2 minutes.

Once the vanilla cake layers are completely cooled, level them to your desired height. Add a swipe of raspberry buttercream onto a cardboard cake circle and place the first cake layer on top. Add about 1 Cup of the raspberry buttercream into a piping bag and snip off about ½ inch opening on the end. Use it to pipe a dam around the edge of the cake layer and frost a thin layer of the raspberry buttercream in the center, then fill the rest with raspberry filling until it reaches slightly below the top edge of the buttercream dam. Place the next vanilla cake layer on top and repeat the process of filling and stacking before placing the final cake layer on top, upside down so that the bottom of the layer becomes the top of the cake. Crumb coat the cake with raspberry buttercream, then refrigerate the cake for 15-20 minutes to let the crumb coat firm up.

To create the design pictured, reserve about ⅔ Cup of raspberry buttercream for the swirls on top and use the rest of the buttercream to frost a smooth buttercream finish. Place the reserved buttercream into a piping bag fitted with Wilton Tip 4B and pipe swirls on top, spaced about ½ inch apart. Place a fresh raspberry in between each swirl.