Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
Whisk together flour, cocoa powder, baking soda, and salt in a bowl.
Beat softened butter, brown sugar, and granulated sugar until light and creamy for 2–3 minutes.
Add egg and vanilla extract to the butter mixture and mix until smooth.
Gradually add dry ingredients into the wet mixture until just combined.
Gently fold in dark chocolate chunks.
Scoop dough into rounds and place on prepared baking sheet.
Press extra chocolate chunks and 2–3 fresh raspberries onto each cookie without overpressing.
Bake for 10–12 minutes until centers are slightly soft.
Immediately sprinkle with flaky sea salt while warm.
Cool cookies on the baking sheet for 5 minutes before transferring to a wire rack.