Prepare a baking sheet by lining it with parchment paper and setting the 4” tart tins onto the baking sheet with space between each.

Cream butter and sugar together in a large bowl with a hand mixer, or in the bowl of a stand mixer on medium speed until combined.

Add in the eggs and beat on medium-high for a couple of minutes.

Add in the oil and vanilla and continue to beat on medium-high speed until the texture is light and fluffy, this should take 4-5 minutes.

Sift the flour, cornstarch, baking powder, and salt into the butter mixture and beat on low speed until combined, scraping the sides and bottom of the bowl as needed.

Scoop about ¼ cup of the dough into one of the 4” tart tins. Using the bottom of a flat measuring cup or drinking glass, press the scoop of dough firmly into the tart tin creating a well in the middle for the filling. Be careful not to press too far, as you don’t want to have any holes in the bottom of the cookie.

After the cookies have chilled, bake them in the preheated oven for 12-15 minutes. The cookies should still look very light in color, just barely starting to turn golden on the edges when you remove them from the oven. If the middles of the cookies have puffed up a little, just give the pan a firm tap or two on the counter to help deflate a little. Let the cookies cool in the tins. When the cookies feel warm but firm it is OK to remove them from the tins and let them finish cooling the rest of the way on a cooling rack.

To make the pastry cream, add the sugar, milk, heavy cream, egg yolks, cornstarch, and salt to a medium-sized saucepan. Whisk until everything is well combined, then slowly over medium (or medium-low) heat bring to a simmer while whisking frequently. As the mixture heats up you will notice it begin to tighten up, keep whisking, once the mixture reaches a boil let it boil for 30-60 seconds then remove from heat and immediately pass the pastry cream through a fine mesh strainer.

Add the butter and vanilla to the warm pastry cream and whisk until the butter has melted and is combined. Cover the pastry cream with plastic wrap, and press the plastic wrap directly onto the surface of the cream to prevent skin from forming on top. Let the pastry cream chill in the fridge until completely cooled.

To make the ganache, pour the chocolate chips into a heat-safe bowl and the heavy cream into a microwave-safe bowl.

Heat the cream in the microwave until it begins to simmer, then remove it from the microwave and pour over the top of the chocolate chips. Let sit for a minute or two then whisk until the chocolate chips have all melted into the cream. Add the butter, and whisk to combine. Set aside to cool for 5-10 minutes. You will want the ganache to be cool enough so that it does not melt your pastry cream filling, but not so cool that it has become firm.

To assemble the cookies, scoop a large spoonful of the pastry cream into the wells of one of the cookies, and use the back of a spoon to spread it around, it is perfect if it is still mounded a bit in the middle.

Use a measuring spoon to spoon some of the ganache over the top of the pastry cream on the cookie, it does not need to be perfect, let the ganache run however it chooses.

It is best to let the cookies chill in the fridge for at least 1-2 hours before serving (although you can absolutely serve right away!) Keep cookies in a single layer in an airtight container for up to 4 days.