A popular libation is brought to layer-cake life with cranberry-infused layers sandwiching a tangy lime curd filling. Soaking the layers with Triple Sec adds moistness (and pleasure) to every bite.
For the lime curd: Stir all of the ingredients together in the top of a double boiler set over medium heat. Cook for 15 to 20 minutes, until the mixture thickens enough to coat a spoon. Remove from the heat and transfer to a small bowl. Place a piece of plastic on top, touching the surface, to keep a skin from forming. Chill until completely set (2 to 3 hours, or overnight).
To make the cake: Preheat the oven to 325°F. Grease and flour two 9" round cake pans.
In a large mixing bowl, beat together the butter, sugar, vegetable oil, baking powder, and salt until the mixture lightens in color and texture.
In a separate bowl, combine the Triple Sec or orange juice, egg whites, and whole egg. With the mixer running at medium-low speed, slowly add this mixture to the butter/sugar mixture.
Add the flour and the yogurt or milk in three additions, alternating between both, until all are used. Scrape down the bowl and add the cranberry juice concentrate. Mix until the batter is uniform in color. For increased pinkness, add a few drops of red food color to the batter. Divide the batter between the prepared pans and level the top with a small offset spatula.
Bake the layers for 30 to 35 minutes, until the top springs back when lightly touched, and the edges just begin to pull away from the sides of the pan. Remove from the oven and cool in the pans on a rack. When cooling cakes for slicing into thin layers, we prefer to have them cool completely in the pan.
To make the soak: In a small saucepan, combine the water and sugar and cook over medium heat, stirring occasionally, until the sugar dissolves. Remove from the heat and stir in the Triple Sec.
For the frosting: Combine the sugar, salt, and water in a medium saucepan, and bring to a boil, stirring occasionally until the sugar dissolves. Boil for 3 to 5 minutes without further stirring, until the syrup reaches 240°F, then pour the hot syrup into the bowl of a stand mixer fitted with a whisk.
Add the cranberry juice concentrate, then sprinkle the meringue powder over the liquid. Whisk slowly until the powder dissolves.
Increase the mixer's speed to medium and beat until the meringue powder mixture begins to get foamy, then turns white and increases in volume. Turn the mixer's speed to high, and beat for 3 to 4 minutes, until the meringue is thick and glossy looking.
Add the butter, a few tablespoons at a time, with the mixer running. The mixture will thin out and look a little curdled for a while; don't let that throw you. Keep the mixer going and add the rest of the butter. If you want more frosting or are planning to serve the cake in a warm environment, beat in the additional 1/2 cup shortening or butter. The frosting will transform itself into a smooth, silky, fluffy mass as you go. If the buttercream just doesn't seem to be setting up, transfer it, bowl and all, to the refrigerator for 30 minutes to chill, then remove from the fridge and resume beating. (Cover the bowl while the buttercream is chilling.)
Add the vanilla and red food coloring, and mix until the frosting is uniformly colored.
To assemble the cake: If the cake layers have a dome on top, use a long, serrated knife to trim them level. If desired, split them horizontally to make 4 layers before soaking. Brush the trimmed side of one layer with 2 to 3 tablespoons of the soaking liquid, then place, trimmed side down, on a serving plate. Brush the second side (formerly the layer's bottom) with more soaking liquid. Once the liquid is absorbed, use a pastry bag and a 1/2" round tip to pile a band of cranberry buttercream around the outside edge of the bottom layer. Refrigerate for 10 minutes to allow the frosting ring to firm up.
Once the frosting ring is firm to the touch, fill the inside of the frosting circle with lime curd. Repeat the soaking process and invert the second layer on top of the first.
Once the layers are stacked, take a heaping cup of frosting and use it to cover the cake all over, top and sides, with a thin layer. It's OK if this looks messy; this is called the crumb coat. Chill the cake for 30 minutes, for the crumb coat to firm up. Once you can touch it without leaving a fingerprint, give the cake another layer of frosting, using any leftover to decorate. Garnish with lime zest or slices, as desired. Chill the cake until 30 minutes before serving.
Store any leftover cake in the refrigerator, well wrapped, for several days.
