Step 1: Make the Custard Filling
In a medium saucepan, whisk together sugar, cornstarch, and salt. Add in the egg yolks and slowly whisk in the milk until smooth. Cook over medium heat, whisking constantly, until the mixture thickens and comes to a gentle boil. Once thickened (like pudding), remove from heat and stir in butter and vanilla. Transfer to a bowl, cover with plastic wrap (press it directly onto the surface to prevent skin forming), and chill for at least 1 hour.

Step 2: Prepare the Cookie Dough
In a bowl, whisk together flour, baking soda, and salt. In a separate large mixing bowl, cream the softened butter with both sugars until light and fluffy, about 3 minutes. Beat in eggs, one at a time, then add vanilla and sour cream. Gradually mix in the dry ingredients until combined. Chill the dough for 30 minutes if it feels too soft.

Step 3: Bake the Cookies
Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper. Scoop out tablespoon-sized balls of dough and place them about 2 inches apart. Bake for 12–14 minutes, or until the edges are set but the centers still look soft. Let cookies cool on the sheet for 5 minutes before transferring to a rack to cool completely.

Step 4: Make the Ganache
Heat the cream in a small saucepan until it just begins to simmer. Pour it over the chocolate and let sit for 1 minute. Add the vanilla and salt, then stir until smooth and glossy. Let cool slightly until thickened but still pourable.

Step 5: Assemble the Cookies
Pair up cookies by size. Pipe or spoon about a teaspoon of chilled custard onto the flat side of one cookie, then gently press the matching cookie on top. Repeat with all pairs.

Step 6: Top with Ganache
Spoon or drizzle a generous dollop of ganache on top of each sandwich. Let them sit until the ganache is set—or sneak one early if you can’t wait. No judgment.