Gently chop blackberries into small pieces and spread on a paper towel-lined plate. Pat very dry to remove excess juice - this prevents soggy cookies. Freeze for 10 minutes while you prep the dough; cold berries hold their shape better.

In a large bowl, beat butter, granulated sugar, and brown sugar on medium-high speed until light and fluffy, about 3 minutes. Scrape down the bowl twice. The mixture should look pale and almost whipped.

Beat in eggs one at a time, mixing well after each addition. Add vanilla and beat until fully incorporated, about 30 seconds.

In a separate bowl, whisk together flour, baking soda, and salt. With mixer on low, gradually add dry ingredients to wet ingredients, mixing just until no flour streaks remain. Do not overmix.

Remove blackberries from freezer. Using a rubber spatula, gently fold in white chocolate chips, then carefully fold in blackberries just until distributed. Some streaking is fine - overmixing will crush the berries.

Cover dough and chill for at least 30 minutes (or up to 24 hours). This firms up the butter and prevents spreading.

Preheat oven to 375F. Line baking sheets with parchment. Scoop dough into 2-tablespoon balls, spacing 2 inches apart. Bake 12-14 minutes until edges are golden and centers look slightly underdone. The berries will look jammy and slightly collapsed.

Cool on baking sheet for 5 minutes, then transfer to a wire rack to cool completely. Cookies will firm up as they cool.