Prepare cake pans by lining the bottoms with parchment paper. You can use either three 8? cake pans, or three 9? cake pans. Preheat the oven to 350 degrees.

In a standing mixer, whisk butter and sugar until light and airy. Approximately 3-5 minutes, stopping at 2 minutes to scrap down the sides.

In a separate bowl, whisk together the egg, egg whites, sour cream, milk and vanilla.

In a third bowl, whisk together the GF flour, baking powder, baking soda, potato starch, guar gum and salt.

Set the mixer to medium/low speed, and alternate adding the wet and dry ingredients to the butter and sugar mixture, starting and ending with the wet ingredients.

Once combined, turn off the mixer. The batter will be smooth and thick.

Divide the batter evenly between the cake pans and use the angled icing spatula to smooth the batter into a flat layer.

Pop the cake pans into the oven and prep your frosting while you wait for these beauties to bake! Bake for 15 minutes, rotate, then bake another 15 minutes. When cakes are golden brown, and you can insert a toothpick into the middle with only crumbs sticking to it – they are ready!

Let cakes cool for 15 minutes before removing from the cake pans. Place the cakes on a cooling rack. They must be completely cool before adding frosting to the cakes.

In a stand mixer fitted with the paddle attachment, beat the softened butter on medium-high speed for about 2 minutes, until pale, smooth, and creamy.

Gradually add the powdered sugar, 1 cup at a time, beating on low after each addition to prevent it from puffing out of the bowl. Scrape down the sides as needed.

Add the heavy cream, vanilla extract, and salt, and mix on low until incorporated.

Increase the mixer speed to medium-high and beat for 5 minutes until the buttercream is light, fluffy, and almost white in color.

If the frosting is too thick, add more cream, 1 tablespoon at a time.

If it’s too soft, add a few extra tablespoons of powdered sugar until it reaches your desired consistency.

Place a small dollop of buttercream in the center of your cake stand or serving plate, and place the first cake layer onto the cake stand or round.

Spread about 1 to 1 ½ cups of buttercream evenly across the first layer using an offset spatula. Start from the center and work outward, keeping the frosting layer even and smooth.

Repeat with the second cake layer, adding more buttercream, then place the third layer on top (bottom side up for a perfectly flat top).

Apply a thin layer of buttercream all over the cake to trap any loose crumbs. Don’t worry if it looks rough — this is just the base layer.

Chill the cake in the refrigerator for 20–30 minutes until the crumb coat is firm to the touch.

Once chilled, add your final layer of buttercream. Use a large offset spatula or bench scraper to spread it smoothly across the top and sides. For a rustic look, leave some visible swirls; for a sleek bakery finish, smooth the sides while slowly turning the cake stand.

Add extra sprinkles around the sides or over the top while the frosting is still soft so they stick easily.

For a polished finish, pipe decorative swirls or rosettes around the top edge using a piping bag fitted with a large star tip (such as Wilton 1M), then sprinkle more funfetti over the top. Or add a few dollops of frosting and press small gluten-free cookies or colorful meringues on top for a celebratory look.

Refrigerate the finished cake for at least 30 minutes before slicing — this helps the layers set and makes clean cuts easier. Before serving, let it sit at room temperature for about 15–20 minutes so the frosting softens and the flavor shines.