Set the butter and egg on the counter 30 to 45 minutes before baking so they reach room temperature, and measure the flour by spooning it into the cup and leveling it off.
In a medium bowl, whisk together the flour, baking soda, and fine salt for about 20 seconds, then set aside.
Beat the softened butter, brown sugar, and granulated sugar on medium speed for 2 to 3 minutes until pale and fluffy, scraping down the bowl once or twice.
Add the egg and vanilla and beat on medium speed for about 1 minute until smooth, then scrape down the bowl again.
With the mixer on its lowest speed, add the flour mixture in two or three additions and mix just until the last streaks of flour disappear.
Fold in the white chocolate chips and dried cranberries by hand with a rubber spatula until evenly distributed.
Cover the bowl tightly and chill the dough in the refrigerator for at least 30 minutes, or up to 2 days.
Place an oven rack in the center position, preheat the oven to 350°F (175°C), and line two rimmed baking sheets with parchment paper or silicone mats.
Use a medium cookie scoop to portion 2-tablespoon mounds about 2 inches apart on the lined sheets, keeping the rest of the dough chilled.
Bake one sheet at a time for 10 to 12 minutes, until the edges are lightly golden and the centers still look soft and puffy, rotating the pan halfway through.
Press a few extra white chocolate chips onto the warm cookies, let them cool on the pan for 5 minutes, then transfer to a wire rack to cool completely.