Preheat oven to 350 degrees F and line three baking sheets with parchment paper. Using a stand mixer fitted with the paddle attachment (or a large mixing bowl with a hand mixer), combine the butter, sugars, egg and vanilla extract and beat on medium speed until creamy and free of lumps.
In a separate bowl, combine the flour, baking soda and salt. Mix well. Gradually add to the butter-sugar mixture and beat on medium speed until just combined. Fold in the chopped rhubarb and raspberries.
Using a medium cookie scoop, drop the dough by 1 1/2 Tbsp onto the prepared baking sheets. Bake in the preheated oven for 9-10 minutes. Let cool. Refrigerate until ready to serve.
Notes
Freeze dough for later use, if desired. Double wrap in plastic wrap or a ziploc bag and freeze for up to 6 months.
Cookies can also be frozen after they have baked and cooled. Store in an airtight container and freeze for up to 4 months for best results.
White chocolate chips can easily be added to the cookie batter. Do not exceed 1 1/2 cups total, if using.