In a small saucepan, combine raspberries, granulated sugar, and lemon juice. Bring to a simmer over medium heat, mashing the raspberries gently with a spoon. Cook for 5-7 minutes until berries have broken down and the mixture is slightly thickened.

In a small bowl, whisk together cornstarch and cold water until smooth. Pour the cornstarch slurry into the simmering raspberry mixture, whisking constantly. Continue to cook for 1-2 minutes until the filling thickens significantly. Remove from heat.

For a smoother filling, press the mixture through a fine-mesh sieve to remove seeds. Let the raspberry filling cool completely to room temperature, then chill in the refrigerator for at least 30 minutes to firm up.

Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans. Line the bottoms with parchment paper.

In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.

In a large bowl of a stand mixer fitted with the paddle attachment (or using an electric hand mixer), cream together the softened butter and granulated sugar on medium-high speed until light and fluffy, about 3-5 minutes. Scrape down the sides of the bowl.

Add eggs one at a time, beating well after each addition until fully incorporated. Beat in the vanilla extract and lemon zest.

In a separate small bowl, whisk together the buttermilk and lemon juice.

With the mixer on low speed, gradually add the dry ingredients to the wet ingredients in three additions, alternating with the buttermilk mixture in two additions, beginning and ending with the dry ingredients. Mix until just combined, being careful not to overmix. A few small lumps are okay.

Divide the batter evenly among the three prepared cake pans. Bake for 28-35 minutes, or until a wooden skewer inserted into the center comes out clean. Cakes should be lightly golden and spring back when gently pressed.

Let the cakes cool in the pans on a wire rack for 10-15 minutes before inverting them onto the wire rack to cool completely. This is crucial before frosting.

In a large bowl of a stand mixer with the paddle attachment, beat the softened cream cheese and butter on medium speed until smooth and creamy, about 2-3 minutes. Make sure there are no lumps of cream cheese.

Gradually add the sifted powdered sugar, 1 cup at a time, mixing on low speed until combined, then increasing to medium-high. Beat until fluffy, scraping down the sides of the bowl as needed.

Stir in the fresh lemon juice, lemon zest, vanilla extract, and salt. Beat on medium-high speed for another 1-2 minutes until the frosting is light and fluffy.

Once the cake layers are completely cool (this can take 1-2 hours), use a serrated knife to level the tops of the cakes if necessary.

Place one cake layer on your serving plate or cake stand. Spread about 1/2 cup of lemon cream cheese frosting evenly over the top, leaving a small border. Create a dam of frosting around the edge of the cake (pipe a ring of frosting with a piping bag or use an offset spatula) to contain the filling.

Spoon half of the chilled raspberry filling within the frosting dam.

Carefully place the second cake layer on top, press gently, and repeat with another layer of frosting and the remaining raspberry filling.

Place the final cake layer on top. Apply a thin layer of frosting all over the cake (sides and top) for a crumb coat. Chill the cake in the refrigerator for 15-20 minutes to set the crumb coat.

Apply the remaining frosting generously over the entire cake, smoothing with an offset spatula or bench scraper. Decorate with fresh raspberries and lemon slices or extra lemon zest, if desired.

Refrigerate the cake for at least 30 minutes before slicing and serving to allow the frosting to set. Bring to room temperature for about 15-20 minutes before serving for best flavor and texture.