Preheat the oven to 350ºF and prepare three 6-inch or two 8-inch cake pans by spraying the sides with cooking spray and fitting a parchment paper circle to the bottoms.

In a large bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside. In a liquid measuring cup, whisk together the milk and lemon juice and set aside.

Using a stand mixer fitted with the paddle attachment, mix together the sugar and lemon zest until fragrant and well combined, about 1 minute. Add the butter and cream it together with the lemon-sugar mixture on medium speed until light and fluffy, about 2 minutes. Add the vegetable oil and beat until combined, light and creamy, about 3 minutes. Scrape down the bowl and paddle as needed.

With the mixer on low speed, add the eggs one at a time and mix until combined. Add the vanilla, then turn the mixer to medium speed and beat for one minute.

Turn the mixer off and add the flour mixture all at once. Mix on low speed until just combined, then slowly pour in the lemon milk mixture. Mix until just combined, about 30 seconds.

In a small bowl, break up the frozen raspberries into smaller pieces and coat them in 1 teaspoon of flour. Then, fold them into the cake batter with a rubber spatula.

Divide the cake batter evenly between the prepared cake pans and bake for 30-35 minutes, until a toothpick inserted into the centers comes out clean or with a few moist crumbs. Cool for 5 minutes before removing the cakes from the pans. Transfer the cake layers to a wire rack to cool completely to room temperature.

In a small bowl, whisk together the cornstarch and water until combined. Add it into a medium saucepan along with the raspberries, granulated sugar, and lemon juice. Set the saucepan over medium heat and stir with a rubber spatula to combine. Cook until the mixture begins to boil, mashing down the raspberries as they soften.

Once the mixture starts to boil, set the timer for 5 minutes while you continue stirring occasionally. Then, remove from heat and transfer to a heat proof container. It won’t seem super thick at this point, but don’t worry – it will thicken significantly as it cools. Let the raspberry filling sit at room temperature for about 20 minutes, then cover and refrigerate for at least 4 hours and up to 1 week.

With a food processor, grind the raspberries into a fine powder. Sift out the seeds and set the powder aside.

Whip the butter using a stand mixer with a paddle attachment on medium-high speed until it’s creamy and light in color (5 minutes). Add the lemon juice, milk, and raspberry powder 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 cake layers have cooled completely, level them to your desired height. Add a swipe of buttercream onto a cardboard cake circle and place the first cake layer on top. Pipe a circle of lemon raspberry buttercream around the edge of the layer, then fill in the center with the cooled raspberry filling until it reaches almost to the top of the dam, then cover the top with more of the lemon raspberry buttercream.

Add the next cake layer on top and continue filling and stacking the cake layers with the raspberry filling and lemon raspberry frosting, placing the final cake layer upside down on top. Crumb coat the cake with more lemon raspberry buttercream, then refrigerate the crumb coated cake for at least 30 minutes to let everything firm up.

To create the design, frost a smooth buttercream finish with the lemon raspberry buttercream and keep the top edge raw. Garnish with a twisted lemon slice and fresh raspberries.