In a medium bowl, gently mash fresh raspberries with a fork, and mix in raspberry jam, a sprinkle of sugar, cornstarch, and lemon juice. Stir until combined and the mixture is slightly thickened. Set the filling aside to allow the flavors to meld while you prepare the cookie dough, giving it about 10 minutes to rest.

In a large mixing bowl, cream together softened unsalted butter, granulated sugar, and brown sugar using a hand mixer on medium speed until light and fluffy—about 3 minutes. Add in the eggs one at a time, followed by vanilla extract, mixing well after each addition.

In a separate bowl, whisk together all-purpose flour, cocoa powder, baking soda, and salt until well blended. Gradually add the dry mixture to the wet ingredients while mixing on low speed. Scrape down the sides of the bowl as needed, and continue mixing until a soft dough forms.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Scoop out a portion of cookie dough (about 2 tablespoons), flatten it in your palm, and place a teaspoon of the raspberry filling in the center. Encase the filling by folding the dough over it and rolling it into a ball.

Place the formed cookies onto the prepared baking sheet, spacing them about 2 inches apart. Bake in the preheated oven for 10-12 minutes, or until the edges appear set and the centers are still soft.

Once baked, remove the cookies from the oven and allow them to cool on the baking sheet for 5 minutes before transferring to a wire rack. Melt the dark chocolate in a microwave-safe bowl, stirring every 30 seconds until smooth. Dip each cooled cookie into the melted chocolate, allowing any excess to drip off.