Preheat your oven to 350°F (175°C). Line a 9x13-inch baking dish with parchment paper, allowing some overhang for easy removal later.
In a large mixing bowl, cream together the soft unsalted butter and lemon cake mix until well combined. The mixture should be thick and slightly crumbly.
Add the large egg to the mixture and stir until fully incorporated, creating a thick batter.
Fold in the sweetened shredded coconut and white chocolate chips, mixing until evenly distributed throughout the batter.
Transfer the batter to the prepared baking dish. Use a spatula to spread it evenly across the bottom, ensuring it reaches all corners.
Bake in the preheated oven for about 15 minutes, or until the edges are slightly golden. The center will still look soft.
Remove the baking dish from the oven and pour the sweetened condensed milk evenly over the baked base, ensuring it covers the entire surface.
Put the dish back in the oven and bake for an additional 20-25 minutes, or until the top is golden and bubbly.
Allow the bars to cool in the pan for at least 30 minutes before lifting them out using the parchment paper. Slice into squares and serve.
