Cream butter and sugars: In a large bowl, beat the butter, brown sugar, and granulated sugar until smooth, light, and creamy. Add the egg and vanilla extract and beat until fully combined.

Mix dry ingredients: In a separate bowl, whisk together baking soda, salt, pudding mix, cake mix, and flour.

Combine: Add dry ingredients to the wet ingredients and beat just until combined. Do not overmix. Dough will be very thick. Gently fold in sprinkles and white chocolate chips.

Chill: Roll dough into 3-tablespoon (60g) balls. Cover and chill for at least 1 hour.

Bake: Preheat oven to 350°F. Line a baking sheet. Place no more than 6 dough balls per pan. Bake for 9 to 13 minutes, until edges are lightly golden and centers are slightly underbaked. About 11 minutes works well.

Cool: Let cookies rest on the pan for 3 to 4 minutes, then transfer to a cooling rack and cool completely.

Beat base: In a large bowl, beat the butter and cream cheese until completely smooth.

Flavor: Mix in vanilla and salt.

Sweeten: Add powdered sugar and beat until smooth. It may seem like it needs more liquid, but keep beating and it will come together. If too wet, add more powdered sugar. If too dry, add a small splash of milk or cream.

Frost: Transfer frosting to a piping bag or plastic bag with the tip snipped off. Pipe a swirl of frosting onto each cooled cookie.

Decorate: Sprinkle with nonpareils if using and gently press a frosted animal cracker into the center of each cookie.

Set: Let frosting set before serving or stacking.