Adjust an oven rack to the middle of the oven. Preheat the oven to 350F [180C]. Line baking sheets with parchment paper.
In a bowl, combine the flour, baking soda, and salt. Set aside.
Pulverize the berries into a powder with a rolling pin or in a food processor
Using a stand mixer, beat butter on medium speed until creamy, about 1 minute. Add sugar and beat on medium speed until light and fluffy, about 2 to 3 minutes. Add egg, egg yolk, and vanilla, and beat on medium speed until combined. Add the flour mixture and beat on low speed until just combined.
Take the dough out and divide into three equal portions, I use a scale. Put one-third of the dough back into the mixer and add the powdered strawberries/raspberries and food coloring, if using. Mix on low speed until totally combined, then remove the dough and wipe out the bowl.
Add another third of dough to the mixer. Add the cocoa powder and mix on low speed until totally combined. If it is too crumbly add a teaspoon of water to bring it together. Try taking it out of the mixer and kneading it a little.
Use a #70 ice cream scoop to form balls of each of the three doughs or pinch a small portion (about 1/2 oz [15 g]) of each, and press them gently together, so they adhere to each other, but colors remain distinct. Shape each into a ball, then roll the ball into sprinkles or granulated sugar. Place 6 or 7 cookies on each sheet pan.
Bake the cookies one pan at a time, rotating halfway through baking. Bake until the sides are set and the cookies are puffed, 10 to 11 minutes. Rap the cookies on the rack as you pull them out so they deflate. This will improve the crackle appearance. If the cookies are baked too long they will have fewer cracks.
Transfer the sheet pan to a wire rack and let the cookies cool for 5 to 10 minutes on the pan, then remove them and let them cool completely on the wire rack.
Cookies can be stored in an airtight container at room temperature for up to 2 days.