Make the ganache: Place the chocolate in a medium bowl. Bring the cream and 1/2 teaspoon salt to a boil in a small saucepan, then pour over the chocolate. Shake the bowl so the chocolate is fully submerged in the cream, then let stand 1 minute. Using a small rubber spatula, slowly stir the chocolate and cream in the center of the bowl until it forms a shiny, smooth ganache. Cover the ganache with plastic wrap and let stand in a cool place in your kitchen until it is thick like pudding, but still pourable and spreadable, up to 2 hours. (See Cook's Note about timing.)
Preheat the oven to 350 degrees F. Cut three 8-inch circles out of parchment paper. Using 2 tablespoons butter, grease and flour three 8-inch round cake pans and line each pan with a round of parchment paper.
Whisk the flour, baking powder and the remaining 1/2 teaspoon salt in a medium bowl until evenly combined. Combine the sugar and remaining 14 tablespoons butter in the bowl of a mixer and beat on medium-high speed until pale and fluffy, about 3 minutes. Add the almond filling, lemon juice and almond and vanilla extracts and beat until smooth. Add the eggs one at a time, beating well after each addition. Add the flour mixture, then stir with a rubber spatula until the batter is combined.
Divide the batter equally among 3 bowls. Add the green, yellow and red food colorings to the batter in their separate bowls, then stir the colorings. Scrape each batter into a prepared cake pan and smooth the top with a small offset spatula or rubber spatula. Bake until the cakes are lightly browned and a toothpick inserted in the middle of each cake comes out clean, 20 minutes.
Transfer the cake pans to a wire rack and let cool 15 minutes. Invert the cakes onto the rack, remove the cake pans and discard the parchment paper rounds. Let the cakes cool completely, about 30 minutes. Flip the cakes right-side up and trim the top of each cake with a serrated knife so that it is flat, if necessary.
Place the green cake on a cake stand and spread 1/4 cup jam evenly over the top. Place the yellow cake over the green and spread the remaining 1/4 cup jam evenly over the top. Place the pink layer on top, then loosely wrap the cake in plastic wrap and refrigerate for 30 minutes.
Unwrap the cake and scrape the ganache on top (the ganache will be the consistency of thick pudding). Using an offset spatula, spread the ganache evenly over the side and top of the cake, leaving a slightly thicker layer on top. Once the ganache is smooth all around, drag the tines of a fork in a wavy pattern across the top of the entire cake. Refrigerate the cake to set the ganache, about 30 minutes. Allow the cake to come to room temperature before serving.
When measuring flour, we spoon into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.) Our test kitchen used regular store-bought chocolate chips for this recipe. If you use a premium variety, the ganache may not need to stand for the full 2 hours.
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