In a medium saucepan, whisk together sugar, cornstarch, and salt. Gradually whisk in the milk until smooth. Cook over medium heat, stirring constantly, until the mixture begins to thicken.

In a separate bowl, lightly beat the egg yolks. Temper the eggs by slowly whisking about a half cup of the warm milk mixture into the yolks. Pour the yolk mixture back into the saucepan.

Continue to cook, whisking constantly, until the cream thickens and comes to a boil. Boil for 1 minute. Remove from heat and stir in the butter and vanilla extract.

Strain the pastry cream through a fine-mesh sieve into a bowl. Press plastic wrap directly onto the surface and refrigerate for at least 2 hours, or until completely chilled and set.

Preheat oven to 375°F (190°C). Line two baking sheets with parchment paper.

In a large bowl, cream together the softened butter and sugar until light and fluffy. Beat in the egg and vanilla extract.

In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined.

Drop rounded tablespoons of dough onto the prepared baking sheets. Bake for 10-12 minutes, until the edges are lightly golden. Let cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.

Place chocolate chips in a heatproof bowl. Heat the heavy cream in a small saucepan until it just begins to simmer. Pour the hot cream over the chocolate chips and let it sit for 5 minutes.

Whisk the ganache until smooth and glossy. Let it cool and thicken slightly.

To assemble, pipe or spoon the chilled pastry cream onto the flat side of one cookie and top with another cookie. Dip the top of the cookie sandwich in the ganache.

Place on a wire rack to let the ganache set. Store cookies in the refrigerator.