Heat the oven to 350°F. Spray an 8" square pan with nonstick spray. Set aside.
Combine the sugar, pumpkin purée, spices, and salt in a medium bowl. Add the egg and stir to combine. Add the flour, baking powder, baking soda, and 1/2 the white chocolate chips. Stir until smooth.
Transfer the batter to the prepared pan and smooth it into an even layer.
Bake until set and the edges are pulling from the sides of the pan, 55 to 60 minutes. Remove from the oven to cool completely while you prepare the white chocolate drizzle.
Place the white chocolate chips in a microwave-safe bowl. Heat for no more than 20 seconds at a time. Each time you heat the chocolate, take it out of the microwave and stir. Continue to heat and stir until the white chocolate is fully melted. Transfer to a sealable plastic bag and snip the end. Drizzle over the cooled cake.