To prepare the cake:
Have all the ingredients at room temperature.
Position a rack in the lower third of the oven and preheat to 325 F. Butter and flour a three-dimensional Christmas tree-shaped pan, tap out the excess flour. Sift together the flour, cinnamon, baking powder, salt, ginger, nutmeg and cloves. In a small bowl, combine the dried fruits and orange liqueur and let soak for at least 15 minutes.
In the bowl of an electric mixer fitted with the flat paddle, beat the butter on medium speed until creamy and smooth, 1 to 2 minutes. Gradually add the brown and granulated sugars and beat until lightened in color and fluffy, about 5 minutes. Stop the mixer occasionally to scrape down the sides of the bowl. Stream in the eggs slowly, beating well after each addition.
Reduce the speed to low and add the flour mixture in three additions, alternating with the milk and beginning and ending with the flour. Beat each addition until just incorporated, stopping the mixer occasionally to scrape down the sides of the bowl. If using the dried fruit, fold it in to the batter until thoroughly combined.
Spoon half of the batter into each half of the prepared pan, spreading a little extra into the top star area of the pan. Place the pan into the oven, taking care to make sure the pan is sitting level on the oven rack. Bake until the cake is golden brown and a toothpick inserted into the center comes out clean, 60 to 70 minutes. Transfer the pan to a cooling rack and let the cake cool for 15 minutes.
In a small saucepan over medium-high heat, combine the orange juice and granulated sugar, and cook, stirring constantly, until the sugar is dissolved. Remove from the heat.
Invert the cake pan onto a wire rack set over a rimmed baking sheet. Lift off the pan. Brush the cake with the warm glaze while it is still warm. Let the cake cool completely before decorating and serving.
*Orange Buttercream is optional, but can be used for assembling the cake. To prepare, have all the ingredients at room temperature.
In the bowl of an electric mixer fitted with the flat beater, beat the butter on medium speed until creamy and smooth, about 2 minutes. Add the vanilla and continue to beat until combined, about 1 minute. Transfer to a bowl and set aside. Thoroughly wash and dry the mixer bowl.
In the clean mixing bowl, with a handheld whisk, whisk together the sugar, egg whites, salt and cream of tartar. Set the bowl over a saucepan with simmering water, making sure the bowl is not touching the water. Whisk constantly until the mixture registers at 140 F on a candy thermometer, 7 to 10 minutes.
Remove the mixing bowl from the saucepan. Attach the mixing bowl to the stand mixer and beat on high speed, using the whisk attachment, until the mixture is cool and soft peaks form, 7 to 10 minutes. Gradually add the butter a spoonful at a time, beating well after each addition until smooth. Add the orange zest and orange liqueur and beat well until thoroughly mixed.
Makes about 2 cups.
Decorating the Christmas Tree Cake
Once the cakes have thoroughly cooled, place both halves of the tree cake back into the tree pan. Trim the surface of the both of the cakes by using a serrated knife to slice off any part of each cake that is taller than the surface of the pan. Carefully remove one of the tree cakes from the pan and place on a clean work surface, leaving the other cake still in the pan.
Spread a thin layer of butter cream, about 1/2 cup, over the exposed (cut) side of the tree cake that is still in the pan. Place the other side of the cake on top of the butter cream and press down gently to secure the two halves. Refrigerate the cake in the tree pan until the butter cream is firm, 20 to 30 minutes.
Spread 1/4 cup of the butter cream in the center of a cake plate. Carefully remove the entire cake from the pan by slightly inverting the pan until the cake comes loose. Place the tree cake upright on the butter cream to anchor it to the cake plate. Loosely cover with plastic wrap and refrigerate until the butter cream is firm, about 30 minutes. Decorate as desired.
Makes 12 to 16 servings.