Set oven to 355°F (180°C). Prepare 2 6-inch (15 cm) round cake pans by lining with parchment paper.

In a large bowl, combine all dry ingredients by whisking until uniform. For enhanced lemon flavor and color, rub the sugar and lemon zest together if desired. Introduce all wet ingredients except the raspberries, whisking until just mixed. Then add raspberries and fold in gently.

Evenly distribute batter among the prepared pans, making sure raspberries are not clumped. Lift and drop pans firmly on the counter to release large air bubbles.

Bake for 43–48 minutes, or until a toothpick inserted in the center emerges without wet batter (a few moist crumbs are acceptable).

Allow cakes to cool in pans for 10 minutes, then move to a wire rack to finish cooling completely.

In a small saucepan over medium heat, mix raspberries, sugar, and a splash of water. Simmer for 5 minutes, crushing berries with the back of a fork.

In a small bowl, combine cornstarch with a splash of water. Stir this slurry into the raspberry mixture and cook until it thickens. For a thicker jam, continue cooking; for a thinner consistency, add a splash of water. Set aside to cool entirely.

In a large mixing bowl or stand mixer, beat butter until it becomes light and fluffy.

Incorporate powdered sugar, berry powder or food coloring, and vanilla. Mix on low until blended, then raise speed to high and whip until light and fluffy. For a softer frosting, add a splash of milk; for more color, adjust as desired. (Refer to note 3 regarding natural food powders.)

If the cakes have domes, level them with a serrated knife. Using the blunt side of a knife, lightly score the top of each cake into quarters. With a sharp knife, remove two 'segments' from each cake (see images in post). These will become the bottom of the heart cake.

Set the first cake layer on a serving plate or cake board. Use a bit of frosting to attach the two cake segments to the opposite side of the cake, creating the top of the heart. Trim as necessary.

For jam filling: spread a thin layer of frosting on the cake surface, pipe a border of frosting around the edge, and spoon jam into the center. If omitting jam, spread about 3/4 cup frosting on the top cake layer.

Place the remaining cake layer on top of the first. Repeat with the cake segments to form a heart shape and trim as needed. Apply a crumb coat by spreading a thin layer of frosting over the sides and top. Refrigerate for at least 20 minutes, until the frosting feels dry to the touch.

Coat the heart cake with a thicker layer of frosting and smooth it using an offset spatula or cake scraper. Decorate the remainder with a piping bag fitted with a star or round tip. Just before serving, garnish with fresh berries.

Keep the frosted heart cake in an airtight container at room temperature for 1 day or in the refrigerator for up to 3 days. If the environment is hot, store it in the fridge. Allow the cake to come to room temperature before serving.