Pour the batter into the prepared pan and use an offset spatula to smooth the top. Bake for 30–35 minutes, or until the top is set and a toothpick inserted into the center of the cake comes out clean with no crumbs. Let the cake cool completely.

Using a stand mixer fitted with the paddle attachment, beat the butter and salt together for about 1 minute until smooth. A hand mixer will also work if you don’t have a stand mixer.

Gradually sift in the powdered sugar a little at a time, mixing on low speed after each addition. Be sure to scrape down the sides of the bowl as needed and continue mixing until everything is fully incorporated.

Add 1 tablespoon of cream while mixing on low speed, then pour in the vanilla extract. Continue beating until the frosting becomes light and fluffy. If you want a thinner consistency, add a little more cream.

Once the sheet cake has completely cooled, use a round cookie cutter that matches the size of your jars to cut out cake circles. If you don’t have a cookie cutter, you can also use the jar lid as a guide so the cake layers fit perfectly inside the jars.

For one layer of cake, stack multiple cake layers directly on top of each other inside the jar without adding frosting between the layers. For layered dot cakes, adding a layer of cake, followed by frosting, then another layer of cake and more frosting. Repeat until the jar is filled, leaving about 1 inch of space at the top for the final frosting layer.

Once the cake layers are assembled, spread a layer of frosting over the top so it reaches close to the rim of the jar. Make sure not to overfill the jar, as you’ll need room for the decorative topping.

To finish, either sprinkle nonpareils over the frosting or dip the top directly into the sprinkles for full coverage. If dipping, pour the sprinkles onto a flat plate or shallow bowl first so the frosting can be evenly coated.