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 the sifted all-purpose flour, baking powder, and salt. Set aside.

In a large bowl, using an electric mixer, cream the softened butter and granulated sugar together on medium speed until light and fluffy, about 3-5 minutes. Scrape down the sides of the bowl as needed.

Add the eggs one at a time, beating well after each addition. Mix in the lemon zest and vanilla extract until just combined.

Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the whole milk, beginning and ending with the dry ingredients. Mix on low speed until just combined, being careful not to overmix. A few streaks of flour are okay.

Divide the batter evenly among the three prepared cake pans. Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean. The tops should be lightly golden and spring back when gently touched.

Let the cake layers cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely. Ensure cakes are completely cool before frosting, which may take 1-2 hours.

In a medium saucepan, whisk together the lemon juice, lemon zest, granulated sugar, and egg yolks until well combined.

Cook the mixture over medium-low heat, stirring constantly with a whisk, until it thickens enough to coat the back of a spoon (about 8-10 minutes). Do not boil.

Remove the saucepan from the heat and stir in the cubed unsalted butter, one piece at a time, until completely melted and incorporated.

Pour the lemon curd through a fine-mesh sieve into a heatproof bowl to remove any lemon zest or cooked egg particles, ensuring a smooth texture. Press a piece of plastic wrap directly onto the surface of the curd to prevent a skin from forming. Chill in the refrigerator for at least 1-2 hours, or until firm.

In a small saucepan, combine the raspberries, granulated sugar, and lemon juice. If using frozen raspberries, there's no need to thaw.

In a small bowl, whisk together the cornstarch and water to create a slurry.

Bring the raspberry mixture to a simmer over medium heat, stirring occasionally. Once simmering, add the cornstarch slurry and cook, stirring constantly, until the mixture thickens, about 3-5 minutes.

Remove from heat and transfer to a bowl. Cover with plastic wrap, pressing it directly onto the surface, and chill in the refrigerator for at least 1-2 hours, or until firm.

In a large bowl, using an electric mixer, beat the softened unsalted butter on medium speed until creamy and pale, about 3-4 minutes.

Gradually add the sifted powdered sugar, 1 cup at a time, beating on low speed until combined after each addition. Once all sugar is added, increase speed to medium-high and beat for another 2-3 minutes.

Add the fresh lemon juice, lemon zest, and vanilla extract. Beat until fully incorporated. Gradually add heavy cream or milk, 1 tablespoon at a time, until the frosting reaches your desired consistency – light, fluffy, and spreadable.

Once the cake layers are completely cool and the fillings are chilled and firm, use a serrated knife to level the tops of the cake layers if necessary, creating flat surfaces.

Place one cake layer on your serving plate or cake stand. Spread a thin layer of lemon buttercream evenly over the top, creating a dam around the edge to hold in the filling. Fill the center with half of the raspberry filling.

Carefully place the second cake layer on top. Repeat the process: spread a thin layer of buttercream, create a dam, then fill the center with the remaining lemon curd.

Place the final cake layer on top. Apply a thin layer of buttercream all over the cake (top and sides) to create a crumb coat. This traps any loose crumbs. Chill the cake in the refrigerator for at least 20-30 minutes to set the crumb coat.

Once the crumb coat is firm, apply the remaining lemon buttercream frosting to the top and sides of the cake. Use an offset spatula or bench scraper to smooth the frosting as desired. Decorate with fresh raspberries and lemon slices, if desired.

Serve immediately or store in the refrigerator. Allow to come to room temperature for 15-20 minutes before serving for best flavor and texture.