Making the Chocolate Cookies: In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set this aside.
In a separate large bowl, beat the softened butter with both brown and white sugars until the mixture is light and fluffy. Then, beat in the egg and vanilla extract until everything is well combined.
With your mixer on low speed, gradually add the dry flour mixture to the wet butter mixture, about half at a time, until just combined. The dough will be a little sticky – that's normal! Fold in the chocolate chips.
Cover the bowl with plastic wrap and place it in the fridge for at least 3 hours (or even overnight). This chilling step is essential for perfect cookies!
When you're ready, preheat your oven to 350°F (175°C). Line your cookie sheets with parchment paper or silicone baking mats.
Scoop the chilled dough into small balls, about 1 to 1.5 tablespoons each (a bit smaller than a golf ball), and space them about 2 inches apart on your prepared baking sheets. Bake for 8-10 minutes, or until the edges are set and the tops look firm. Let them cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
Whipping Up the Raspberry Cream Cheese Frosting: Pop the raspberries into a blender or food processor and pulse until they're super smooth.
To get that silky-smooth frosting, push the raspberry puree through a fine-mesh sieve or metal sifter to remove all the tiny seeds. Use the back of a spoon to help it along.
Pour the seedless puree into a small saucepan over low heat. Gently bring it to a simmer, stirring often, and let it boil down until it thickens into a jam-like consistency. You'll want just a few concentrated tablespoons. Make sure to let it cool completely before using!
In a large bowl, beat the softened butter until it's light and creamy. Add the cream cheese and beat until the mixture is smooth and lump-free.
With the mixer on low speed, gradually add about 1 cup of powdered sugar and the salt until combined.
Now for the lovely flavor! Stir in 1 tablespoon of your completely cooled, thick raspberry puree. Continue adding the remaining powdered sugar, about 1/2 cup at a time, until your frosting reaches a beautiful, spreadable consistency. If you desire a stronger raspberry flavor, you can mix in another tablespoon of puree.
Assembling Your Sweetheart Sandwiches: Once your chocolate cookies are fully cooled, spread about 1 tablespoon of the luscious raspberry frosting onto the flat side of one cookie. Gently press a second cookie on top to create a perfect sandwich.
Repeat with the remaining cookies and frosting. Store these delicious cookies in the fridge to keep the frosting perfectly firm.