In a mixing bowl, cream butter and sugar together until light and fluffy, about 3 minutes. Scrape down the bowl halfway through mixing.

Add sour cream, egg, and vanilla. Beat until fully combined.

Add 1 cup flour and baking soda. Mix just until combined.

Add remaining 2 cups flour and mix until dough forms.

Cover and refrigerate dough for 30 minutes.

Preheat oven to 375°F. Line baking sheets with parchment paper.

Lightly flour a flat surface and rolling pin.

Roll dough to about ¼-inch thickness.

Cut fish shapes using a cookie cutter.Place cookies on prepared baking sheet without touching.

Bake 5–7 minutes or until bottoms are lightly golden.

Let cookies rest on baking sheet for 5 minutes before transferring to a cooling rack.

Cool completely before decorating.

In a mixing bowl, combine powdered sugar, meringue powder, and 2 tablespoons water.

Mix on low speed until mixture forms a thick paste.

Slowly add remaining water 1 tablespoon at a time until icing reaches flooding consistency.

Continue mixing about 5 minutes until smooth.

Add gel food coloring and mix until evenly colored.

Transfer icing to a piping bag and snip a small opening.

Pipe an outline around each cookie. Fill center with icing.

Use a toothpick to spread icing evenly and remove air bubbles.

Add sprinkles while icing is still wet.

Allow icing to dry completely before serving or storing.