Preheat oven to 350°F (175°C). Line a 9×9-inch pan with aluminum foil (leave an overhang for easy removal) and spray the foil with cooking spray; set the pan aside.

In a large bowl, add ½ cup very soft unsalted butter. Using a handheld electric mixer on medium-high speed, beat the butter until smooth and creamy (about 30–45 seconds).

Add the entire 16.50-ounce box lemon cake mix and 1 large egg to the bowl. Beat on low speed until combined into a very thick dough. If the mixture looks crumbly after a few minutes, squeeze a small amount with your fingers — it should come together.

Divide the dough roughly as follows: use between half and two-thirds of the dough for the base and set the remaining dough aside for the topping.

Press the base portion of the dough evenly into the prepared pan, forming a smooth, even layer (use your hands or the back of a spoon). Set the reserved dough aside.

Evenly sprinkle 1 cup sweetened shredded coconut over the dough base, then evenly sprinkle 1 cup white chocolate chips on top of the coconut.

Measure out about 8 ounces (roughly 60% of a 14-ounce can) of sweetened condensed milk and drizzle it evenly over the coconut and white chocolate chips.

Break the reserved dough into approximately 1-inch pieces and distribute them over the top so some of the filling remains exposed. Lightly press the pieces so they adhere but do not try to completely cover the filling.

Bake in the preheated oven for 27–30 minutes, until the edges are set and light golden brown and the center is mostly set (the center can be slightly jiggly). Watch closely in the final minutes to prevent the coconut and white chocolate from burning.

Remove the pan from the oven and place on a wire rack. Allow to cool at room temperature for at least 2–3 hours (overnight is better) or until the bars have firmed up enough to lift from the pan and cut. Use the foil overhang to remove the bars before slicing.