Line an 8-inch square baking dish with foil, leaving a 2-inch overhang on two sides; lightly coat the foil with cooking spray. Bring 1 inch of water to a simmer in a medium saucepan over medium heat.
Combine the white chocolate, cookie butter and salt in a large heatproof bowl and set it over the saucepan (do not let the bowl touch the water). Cook, stirring frequently, until the mixture is completely melted and smooth. (Reduce the heat if steam sputters around the bowl — you want to prevent any water from getting in the chocolate and causing it to seize.)
Remove the bowl from the pan and stir in the molasses. Wipe the water from the bottom of the bowl, then pour the mixture into the prepared baking dish. Spread evenly with a rubber spatula and top with white nonpareils. Let set at room temperature, 1 to 2 hours. Cover with plastic wrap and refrigerate until chilled but not hard, about 30 minutes.
Lift the fudge out of the baking dish and peel off the foil. Cut into 1-inch squares with a sharp knife. (If the fudge is too hard or starts to crack when cut, let soften slightly at room temperature, 10 to 20 minutes.)