Take a bite of this orange-cream cake and you’ll undoubtedly think of a Creamsicle, albeit one made with fresh orange juice and vanilla extract. It’s wonderful in the winter, when oranges are plentiful, but it’s also a refreshing dessert for a hot summer night when you don’t have any change, or the ice cream truck is miles away.–Trish Boyle
Position a rack in the center of the oven and preheat the oven to 350°F (175°C). Grease the bottom and sides of a 9-inch round cake pan. Dust the pan with flour.
Sift together the cake flour and salt into a medium bowl. Whisk to combine, and set aside.
In the bowl of an electric mixer, whisk together the eggs and sugar by hand. Set the bowl over a saucepan of simmering water (make sure that the bottom of the bowl does not touch the water) and heat, whisking constantly, until the eggs are warm. Transfer the bowl to the electric mixer stand and, using the whisk attachment, beat on high speed until the mixture has tripled in volume, about 8 minutes. Reduce the speed to low and beat in the orange zest and vanilla extract.
Sift one-third of the flour mixture over the batter and gently fold it in with a rubber spatula. Sift in the remaining flour mixture in two more additions, again folding in gently. Put the melted butter in a small bowl, scoop about 3/4 cup of the cake batter into the bowl, and stir until blended. Fold this mixture into the remaining cake batter. Scrape the batter into the prepared pan.
Bake the cake for 18 to 22 minutes, until the top springs back when lightly touched and a tester inserted into the center comes out clean. Cool the cake in the pan on a wire rack for 15 minutes.
Invert the cake onto the wire rack and cool completely.
In a small saucepan, combine the sugar and water and bring to boil, stirring to dissolve the sugar. Remove the pan from the heat and stir in the orange juice and liqueur, if using. Set aside to cool.
Pour the water into a medium saucepan and sprinkle the gelatin over it. Let the gelatin soften for 5 minutes.
Whisk the orange zest, orange juice, lemon juice, sugar, and yolks into the gelatin. Place the pan over medium heat and cook, whisking constantly, until the mixture thickens and reaches 180°F (82°C) on an instant-read thermometer. Pour the mixture through a fine-mesh sieve into a medium bowl. Stir in the orange liqueur, if using, and vanilla extract.
Set the bowl containing the orange mixture in a large bowl filled about one-third of the way with ice water (be careful that the water doesn’t splash into the orange mixture). Stir the orange mixture frequently until it is completely cool, about 10 minutes.
In the bowl of an electric mixer, using the whisk attachment, whip the heavy cream at medium-high speed to firm peaks. Fold in the orange mixture. (The mousse should be used immediately.)
Using a long serrated knife, cut the cake horizontally into 2 layers. Place a cake layer cut side up in the bottom of a 9-by-3-inch pan, centering it in the pan. Generously brush the cake with half of the orange syrup. Scrape half of the mousse onto the cake and, using a small offset metal spatula, spread it into an even layer, letting the mousse fill the gap between the cake and the side of the pan. Center the remaining cake layer, cut side up, on top. Brush with the remaining orange syrup. Scrape the remaining mousse on top and spread it into an even layer as before.
Refrigerate the cake for at least 3 hours, until set.
Using a sharp paring knife, remove the peel of each orange in vertical strips, trying not to include any of the bitter white pith. If any of the pith remains, place each strip, pith side up, on a cutting board and use the paring knife, with the blade parallel to the board, to carefully slice it off. Cut the zest into fine julienne strips.
Half fill a medium saucepan with water and bring it to a boil. Add the strips of zest, reduce the heat to a simmer, and simmer for about 15 minutes. Drain and rinse the zest.
In the same saucepan, combine the sugar, water, and cream of tartar and bring to a boil, stirring constantly to dissolve the sugar. Add the zest, cover the pan, and reduce the heat to low. Let the zest simmer for another 15 minutes. Remove the pan from the heat and cool completely.
The zest can be stored in its syrup in an airtight container for up to a month. When you are ready to use it, drain it well and toss it in the granulated sugar, breaking up any lumps of sugar with your hands. Spread the zest out on a baking sheet and let dry at room temperature for at least 2 hours before using.
Run a thin-bladed, sharp knife under hot water and wipe dry. Run the knife between the cake and the side of the springform pan to loosen the cake; reheat the knife as necessary. Remove the side of the pan. Use a small metal spatula to smooth the mousse on the sides of the cake if necessary. Refrigerate the cake if not serving immediately.
To serve, garnish the top of the cake with the whipped cream (either piped or dolloped) and candied orange zest. Slice the cake using a thin-bladed, sharp knife. Store in the refrigerator, loosely covered, for up to 3 days.