Step 1: Prepare the Orange Cake

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and line the bottom of an 8-inch or 9-inch round cake pan with parchment paper for easy removal.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
3. Cream Butter and Sugar: In a large mixing bowl, use an electric mixer to beat the softened butter, sugar, and orange zest until light and fluffy, about 2-3 minutes.
4. Add Eggs: Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
5. Add Orange Juice and Sour Cream: On low speed, add the orange juice and sour cream, mixing until just combined.
6. Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix, as this can make the cake dense.
7. Bake: Pour the batter into the prepared pan and smooth the top. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
8. Cool: Remove the cake from the oven and allow it to cool in the pan for 10 minutes. Then, transfer to a wire rack to cool completely before frosting.

Step 2: Make the Marshmallow Frosting

1. Combine Ingredients: In a heatproof bowl (or the bowl of a stand mixer), combine the egg whites, granulated sugar, and cream of tartar.
2. Create a Double Boiler: Place the bowl over a pot of simmering water, making sure the bottom of the bowl does not touch the water. Whisk continuously until the sugar dissolves and the mixture reaches 160°F (70°C) on a candy thermometer, about 4-5 minutes.
3. Whip the Frosting: Remove the bowl from heat and transfer to a stand mixer fitted with a whisk attachment (or use a hand mixer). Beat on high speed until stiff, glossy peaks form and the mixture cools to room temperature, about 5-7 minutes. Add the vanilla extract and mix briefly to combine.

Step 3: Assemble and Frost the Cake

1. Frost the Cake: Once the cake is completely cool, use an offset spatula to spread a thick layer of marshmallow frosting over the top and sides of the cake. Swirl the frosting to create a snowy, fluffy texture.
2. Add Garnishes: Sprinkle red and white holiday sprinkles over the frosting for a festive touch. If desired, dust the top lightly with powdered sugar for extra “snow.”

Step 4: Make the Reindeer Toppers

1. Create the Reindeer Faces: Take each marshmallow and insert a lollipop stick or decorative straw through the center.
2. Add Antlers: Break mini pretzels in half and press them into the top sides of each marshmallow to resemble antlers.
3. Decorate the Faces: Use a small dab of frosting or melted chocolate to attach the red candy nose and black candy or chocolate chip eyes. Repeat for each reindeer topper.

Step 5: Decorate and Serve

1. Insert Reindeer Toppers: Insert the reindeer toppers into the cake, spacing them evenly for a whimsical look.
2. Serve and Enjoy: Slice and serve the cake at room temperature. Enjoy the citrusy, fluffy goodness of this holiday treat with family and friends.