Make the Vanilla Sheet Cake

1. Preheat the oven to 350°F. Lightly grease a 13×18-inch rimmed half-sheet pan. Line the bottom with parchment paper and lightly grease the parchment.
2. In a medium bowl, whisk together the flour, baking powder, baking soda and salt.
3. In a large mixing bowl, beat the softened butter and granulated sugar on medium speed for two to three minutes, until light and fluffy.
4. Add the eggs one at a time, beating well after each addition. Mix in the vanilla.
5. In a separate bowl or measuring cup, whisk together the milk and sour cream.
6. With the mixer on low, add the flour mixture in three additions, alternating with the milk mixture in two additions. Begin and end with the dry ingredients. Mix just until combined, scraping the bowl as needed.
7. Spread the batter evenly in the prepared pan, using an offset spatula to reach the corners and level the surface.
8. Bake for approximately 16 minutes, or until the center springs back when lightly touched and a toothpick comes out clean or with a few moist crumbs.
9. Cool the cake completely in the pan.

Cut the Cake Rounds

1. Select two round cutters that match the interior widths of the tapered cups.
2. Cut eight larger rounds and eight smaller rounds. Cut the larger rounds first, then use the smaller cutter in the remaining spaces.

Make the Vanilla Buttercream

1. Beat the softened butter on medium speed for two to three minutes, until smooth and creamy.
2. Gradually add the confectioners’ sugar, mixing on low until incorporated.
3. Add the vanilla, salt and 4½ tablespoons of cream or milk.
4. Beat on medium-high for two to three minutes, until smooth and fluffy.
5. Add more cream or milk a little at a time, if needed. The buttercream should be spreadable but slightly firm.

Assemble the Dot Cakes

1. Place one smaller cake round in the bottom of each 8-ounce clear cup.
2. Add a generous layer of buttercream and spread it to the edges.
3. Place one larger round over the frosting and press down gently.
4. Add enough buttercream to fill the cup completely to the rim.
5. Use a small offset spatula to make the buttercream flat and flush with the rim.

Add the Nonpareils

1. Pour the nonpareils into a shallow bowl in a deep, even layer.
2. Turn one cup completely upside down and gently press the frosted surface into the nonpareils.
3. Lift the cup straight up and turn it right side up.
4. Press additional nonpareils onto any uncovered areas and repeat with the remaining cakes.

Chill and Serve

1. Refrigerate the assembled cakes for at least two hours.
2. Serve chilled for the firmest topping, or let the cakes stand at room temperature for 10 to 15 minutes for a softer texture.