Line an 8x8-inch (20x20 cm) square baking pan with parchment paper, leaving overhang on two sides to create handles for lifting. Lightly spray the parchment with nonstick spray if desired.
1 sheet parchment paper, nonstick spray
Roughly chop 2 to 2 1/2 cups of frosted circus animal cookies into a mix of larger chunks (about peanut-size) and smaller crumbly bits. Set aside.
2 cups frosted circus animal cookies
In a large microwave-safe bowl, combine the white chocolate chips and sweetened condensed milk. Microwave on 50% power for 45–60 seconds, then stir well. Continue microwaving in 20–30 second bursts at 50% power, stirring after each interval, until the mixture is thick, glossy, and completely smooth with no visible chocolate chunks.
3 cups white chocolate chips, 14 oz sweetened condensed milk
Stir the vanilla extract and fine sea salt into the melted chocolate mixture until fully combined to create the fudge base.
1 teaspoon pure vanilla extract, 1/4 teaspoon fine sea salt
For a pink circus fudge variation, add 1–2 drops of pink gel food coloring to the warm fudge base and stir until evenly colored. Alternatively, pour out half of the uncolored fudge, tint the remaining half pink, and use the two colors for a swirl effect later.
1 drop pink gel food coloring
Let the fudge base cool for 2–3 minutes so it is warm but not piping hot. Gently fold in the chopped circus animal cookies and rainbow sprinkles with a spatula until evenly distributed, being careful not to overmix or crush the cookie pieces too finely.
2 cups frosted circus animal cookies, 1/4 cup rainbow sprinkles
Scrape the fudge mixture into the prepared baking pan and spread it into an even layer. For a pink-and-white swirl, dollop spoonfuls of each color into the pan, then gently swirl with a butter knife without overmixing so the colors remain distinct.
Press a few extra whole or halved frosted circus animal cookies into the surface of the fudge. Sprinkle additional rainbow sprinkles over the top if desired.
extra frosted circus animal cookies, 1/4 cup rainbow sprinkles
Refrigerate the pan of fudge for 1–2 hours, or until firm to the touch. If your kitchen is very warm, let the pan sit at room temperature for about 15 minutes before refrigerating to minimize condensation.
Once fully set, lift the fudge out of the pan using the parchment paper handles and transfer to a cutting board. Use a sharp knife to cut into 1- to 1.5-inch squares, wiping the blade with a warm, damp cloth between cuts for clean edges. Serve and enjoy.