Follow the steps in order for the best result.
Begin by preheating your oven to 350 degrees F (175 degrees C). Prepare a 9x9 inch baking pan by greasing it thoroughly to prevent sticking.
In a double boiler, gently melt the chopped white chocolate and softened butter together. Ensure the water is barely simmering. Stir the mixture occasionally until it becomes smooth and fully melted. Remove from heat and allow it to cool slightly.
In a large mixing bowl, use an electric mixer to beat the eggs until they become light and foamy. Gradually add the white sugar and vanilla extract while the mixer is running, incorporating them well into the egg mixture. Slowly drizzle in the melted white chocolate mixture, continuing to mix until everything is well combined.
In a separate bowl, whisk together the all-purpose flour and salt. Gently fold this dry mixture into the wet white chocolate mixture using a rubber spatula or a wooden spoon. Be careful not to overmix; fold until just combined. Finally, fold in the semi-sweet chocolate chips, distributing them evenly throughout the batter.
Pour the blondie batter into the prepared baking pan and spread it evenly to ensure uniform baking.
Bake in the preheated oven for approximately 25 minutes, or until a toothpick inserted into the center comes out clean. This indicates that the blondies are perfectly baked.
Once baked, remove the pan from the oven and let it cool completely on a wire rack before cutting the blondies into bars. This will allow them to set properly and make cutting easier.