Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper. In a large mixing bowl, combine the granulated sugar and lemon zest, rubbing them together with your fingers to release the zest’s fragrant oils. Add the butter and brown sugar, then beat on medium-high speed for 3 to 4 minutes until the mixture is light and fluffy. Next, add the egg yolk, vanilla extract, and lemon juice, mixing until well combined. Gradually add the salt, baking powder, baking soda, and flour, mixing slowly and carefully to avoid overworking the dough—stop when you still see some patches of flour in the bowl. Chop or gently break apart the frozen raspberries into small pieces, then fold them lightly into the dough, being careful not to overmix to prevent the juice from bleeding throughout. Using a 3-tablespoon cookie scoop, portion the dough into large balls and place five on each prepared baking sheet. Sprinkle each dough ball with a pinch of flaked salt. Bake the cookies for 12 to 15 minutes, until the edges are a light golden brown. After baking, allow the cookies to rest on the baking sheets for a few minutes to finish cooking and set in the center before transferring them to a cooling rack.