Bursting with berries, these colorful blondies are an easy, portable dessert. For a bigger crowd, double the recipe and use a 9" x 13" pan.
Preheat the oven to 350°F. Grease an 8" square pan, line it with parchment paper or foil, and grease the paper or foil.
Melt the butter and sugar together. Add the vanilla and let the mixture cool to lukewarm.
Mix in the eggs, one at a time. Stir in the flour, baking powder, and salt. Mix until combined, then fold in the berries.
Scoop the batter into the prepared pan, and bake for 30 to 35 minutes if using fresh berries; 35 to 40 minutes for frozen, until the edges of the bars just begin to pull away from the pan.
Remove from the oven and cool for 20 minutes before lifting carefully out of the pan (see "tips," below) and setting on a rack to cool completely.
Store, refrigerated, for up to 1 week.
Cutting bar cookies in the pan can be cumbersome. We solve the problem by lining our bar pans with parchment before baking, with the extra paper coming up two sides of the pan. Secure the paper to the pan with metal spring clips. After the bars are baked and cooled, free up the short ends with a dull knife or bowl scraper. Unclip the paper and use it to lift the bars up and out of the pan to a cutting board, where they're much easier to portion neatly.
