Grab a baking sheet and arrange the tart tins on it with space between each tin. Set aside.
Add the butter and sugar to a large mixing bowl and beat on medium speed until combined. Add the eggs and mix to combine. Add the vegetable oil and vanilla and beat on medium-high speed until light and fluffy, about 4-5 minutes.
After the wet ingredients are mixed, sift the flour, cornstarch, baking powder, and salt into the bowl. Then mix on low speed until just combined. Scrape the side of the bowl as needed.
Add 1/4 cup of cookie dough to each tart tin. Use the bottom of a drinking glass or measuring cup to press the dough firmly into the tin so the sides are filled, there is a well in the center, and the cookies have an even thickness all the way around.
Chill the cookie dough for 15 minutes in the fridge. Preheat your oven to 350 degrees F.
Bake for 12-15 minutes or until the edges just start to turn golden. The cookie itself should be very light in color.
Remove from the oven and give the pan a firm tap or two on the counter to deflate the cookies a little. Cool in the tins until the cookies have firmed up and are cool to the touch. Transfer to a wire rack to cool completely.
Pastry Cream
Add the milk, heavy cream, egg yolks, sugar, cornstarch, and salt to a medium saucepan and whisk everything together.
Place the pan over medium heat and bring to a simmer, whisking frequently. The mixture will thicken as it cooks. Once the cream gets to a boil, let it bubble for 3-60 seconds. Remove from the heat and immediately press the cream through a fine mesh sieve into a heat-safe bowl.
Whisk in the butter and vanilla until melted and combined. Cover with plastic wrap and press it directly onto the surface of the cream. (This will prevent skin from forming.) Chill in the fridge until completely cooled.
Chocolate Ganache
Pour the chocolate chips into a heat-safe bowl. Pour the heavy cream into a microwave-safe bowl or glass measuring cup. Heat the cream for 1-2 minutes or until it starts to simmer.
Pour the hot cream over the chocolate chips and let that sit for 2 minutes. Then whisk the chocolate and cream until smooth and shiny. (It will look broken for a bit, just keep whisking.)
Whisk in the butter until melted and combined. Set aside to cool for 5-10 minutes. (You want to cool enough that it won't melt the pastry cream, but not so cool that it's firm.)
Assembly
Add a large spoonful of pastry cream to the center of a cookie tart. Use the back of the spoon to spread it into an even layer (or a bit mounded in the center).
Spoon some of the ganache over the top of the pastry cream letting it run how it chooses. (You want to be able to see the cream under the chocolate.) Chill for 1-2 hours before enjoying for the best flavor.
Notes
Recommended serving size from Crumbl is 1/4 cookie. These cookies are plenty big enough to each cut one into quarters, just like the ones from the store.
If desired, you can make the pastry cream 1-2 days ahead of time. Just keep it in the fridge with the plastic wrap pressed onto the top of the cream.
Store any extra cookies in a single layer in an airtight container in the fridge for up to 4 days.
If you don't have/want to buy tart tins:
Scoop ¼ cup of the cookie dough into the palm of your hand. Roll the dough into a ball and place it onto a parchment paper-lined cookie sheet. Leave plenty of room between the cookies.
Press the cookie dough ball down to about ½-inch thickness using the bottom of a measuring cup or drinking glass. Find a second flat-bottomed measuring cup or glass that is slightly smaller than the first one and use it to press down into the flattened cookie to make a well large enough for the filling (just don't press so hard that you break through the bottom). Press the edges of the cookie around the cup to fix any cracked edges. Then bake and continue on as directed. (see image here)