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

In a large bowl, whisk together the flour, granulated sugar, baking powder, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream the softened butter on medium speed until light and fluffy (about 2-3 minutes).

Add eggs one at a time, beating well after each addition. Scrape down the sides of the bowl as needed. Stir in the vanilla extract.

Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined, being careful not to overmix.

Divide the batter evenly into three separate bowls (about 2 1/4 cups each).

For the Vanilla Layer: Leave one bowl of batter as is. Pour into one prepared cake pan.

For the Chocolate Layer: In a small bowl, whisk together the cocoa powder and hot water (or coffee) until smooth. Stir this mixture into the second bowl of batter until fully combined. Pour into the second prepared cake pan.

For the Strawberry Layer: Stir the strained strawberry purée and a few drops of food coloring (if using) into the third bowl of batter until fully combined. Pour into the third prepared cake pan.

Bake the cake layers for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cakes cool in the pans for 10-15 minutes, then invert them onto a wire rack to cool completely.

While cakes cool, combine 1 cup hulled strawberries and optional 1 tbsp sugar in a food processor or blender. Process until smooth. Strain through a fine-mesh sieve to remove seeds, if desired. Set aside.

In the bowl of a stand mixer with the paddle attachment, beat the softened butter on medium-high speed until creamy, about 3-4 minutes.

Gradually add the sifted powdered sugar, 1 cup at a time, mixing on low speed until combined, then increase to medium. Add heavy cream (or milk), vanilla extract, and salt. Beat on high speed for 3-5 minutes until the frosting is light, fluffy, and smooth.

Once cakes are completely cool, use a serrated knife to level the tops of each cake layer if needed, creating flat surfaces.

Place one cake layer (e.g., chocolate) on your serving plate or cake stand. Spread about 3/4 cup of buttercream evenly over the top.

Drizzle or spread half of the prepared strawberry purée over the buttercream layer. Be mindful not to apply too much liquid.

Carefully place the second cake layer (e.g., vanilla) on top. Repeat with another 3/4 cup of buttercream and the remaining strawberry purée.

Place the final cake layer (e.g., strawberry) on top. Frost the top and sides of the entire cake with the remaining buttercream. Use an offset spatula to smooth the frosting.

Chill the cake in the refrigerator for at least 30 minutes before slicing to allow the frosting to set and the layers to firm up.