I start by whisking together the milk, sugar, and cornstarch in a small saucepan. I heat it over medium heat until it starts to thicken slightly. Then, I quickly whisk the egg yolk in a separate bowl and temper it by slowly pouring in a bit of the hot milk mixture while stirring constantly.

I return it all to the saucepan and cook for another 2 minutes until thick. I stir in the vanilla extract and let it cool completely.

In a bowl, I whisk together the flour, baking powder, baking soda, and salt. In another bowl, I cream the butter and sugars until light and fluffy. I beat in the egg and vanilla, followed by the sour cream. I slowly add the dry ingredients and mix until just combined.

I preheat my oven to 350°F (175°C) and line my baking sheets with parchment paper. Using a cookie scoop, I drop mounds of dough onto the sheets, spacing them about 2 inches apart. I bake them for 10–12 minutes until the edges turn golden. I move them to a cooling rack to cool completely.

Once the cookies cool, I spread a spoonful of the cooled pastry cream onto the flat side of one cookie. Then I top it with another cookie to make a sandwich.

I melt the chocolate chips and heavy cream together in the microwave in 20-second bursts, stirring until smooth. I drizzle the chocolate glaze over the tops of the cookie sandwiches.