Preheat the oven to 350°F/180°C. Line an 8-inch (23 cm) square pan with aluminum foil or parchment, then coat with nonstick spray.

Melt the butter in a small saucepan over low-medium heat or in the microwave. Remove from heat, pour into a large mixing bowl (if melted on stovetop) and cool until just warm.

Meanwhile whisk the flour, baking powder, and salt together in a small bowl to aerate and combine; set aside.

Whisk the brown sugar and vanilla into the melted butter until combined, then whisk in the egg. Fold in the flour mixture until a few floury streaks remain, then fold in the white chocolate and dried cranberries just until combined. Scrape into prepared pan and smooth top with a small offset spatula.

Bake for 20 to 25 minutes or until a toothpick inserted in the center shows a few moist crumbs clinging when removed. The bars should be light golden brown, slightly puffed, and the edges will have just begun to come away from the sides of the pan. Cool in the pan on a rack for 10 minutes, pull up on foil or parchment to remove bars from pan, then cool bars on rack completely. Peel away foil or parchment and cut into a 5 by 5 grid (25 bars). Store at room temperature for up to 3 days in an airtight container in single layers separated by aluminum foil parchment paper.