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.

Use the angled icing spatula to smooth the batter into a flat layer in the sheet cake pan.

Pop the sheet pan into the oven and prep your frosting while you wait for this beauty to bake! Bake for 20-25 minutes. When the cake is a golden brown, and you can insert a toothpick into the middle with only crumbs sticking to it – they are ready!

Let cakes cool for 30 minutes before using the biscuit cutters to make the rounds. I use the two middle sizes in this biscuit cutter set. Cut out an even amount of each size. Place the rounds on a cooling rack. They must be completely cool before adding to mason jars!

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.

Prepare your piping bag and put the first round of frosting in the bag!

Once all of your cake rounds have been cut and have cooled for 30 minutes, you are ready to assemble!

Cut each round in half using a bread knife.

Place the smaller round in the bottom of the mason jar.

Pipe a layer of frosting.

Place the larger round on top of the frosting.

Pipe another layer of frosting and use an offset spatula to smooth the frosting flat.

Pour your desired sprinkles into a bowl and flip the mason jar so the sprinkles get stuck to the top layer of frosting.

Top with lids or leave as is!