Note: this cookie dough needs time (30-60 minutes) to chill in the fridge, plan accordingly.
Set the butter in a heatproof bowl and warm it in the microwave until it’s half melted. Add the sugars, light brown and granulated, and whisk to combine. Return the bowl to the microwave for a full minute then add the vanilla and salt, and whisk the mix until it turns glossy and thick. Let cool.
Add the egg and whisk until incorporated. Switch to a rubber spatula and add the baking powder, baking soda, and flour. Stir then add the white chocolate chips and combine.
Separately, toss the black raspberries with the starch. Add them to the dough and now very carefully, using a folding motion (not stirring), incorporate the berries. Don’t overdo it, stop when they are fairly evenly distributed.
Set the bowl in the fridge for about 30 minutes so the dough can chill.
Preheat the oven to 350 F and line two cookie sheets with parchment paper.
Use a 2 tablespoon cookie scoop to portion the dough onto the prepared pans: leave enough space between each dough ball as these will spread.
Bake the cookies until the edges are golden brown and the middles are matte (not shiny or doughy looking), for about 12-13 minutes. If they are misshapen, use an upside down container to scoot them into a circle.
Store leftover cookies on a plate or tray, covered only with a tea towel.