Line a baking sheet with parchment paper and arrange the 4-inch tart pans with space between them.
Beat the softened butter and granulated sugar together until fully blended and smooth.
Add the eggs and continue mixing until incorporated and slightly thickened.
Pour in the oil and vanilla extract, then beat until the mixture becomes light and fluffy in texture.
Combine the flour, cornstarch, baking powder, and salt, then mix into the dough on low speed until just blended.
Portion about ¼ cup of dough into each tart pan and press firmly to form a well without breaking through the base.
Chill the formed cookie cups for 15 minutes while preheating the oven to 350°F.
Bake the chilled cookies until pale with lightly golden edges, then tap gently if centers puff and cool before removing from pans.
Whisk sugar, milk, cream, egg yolks, cornstarch, and salt together in a saucepan and heat slowly while whisking until thick and boiling.
Strain the hot pastry cream, then whisk in butter and vanilla until smooth before covering and chilling fully.
Place chocolate chips in a bowl and pour hot cream over them, letting it sit briefly before whisking smooth with butter
Spoon chilled pastry cream into each cookie cup and gently spread while keeping a soft mound.
Drizzle ganache over the cream, allowing it to naturally flow over the surface.
Refrigerate assembled cookies for at least one hour before serving for best texture.
Notes
A nostalgic handheld dessert that layers soft vanilla cookie, silky pastry cream, and glossy chocolate. These cookie cups feel special yet comforting, like a bakery treat made at home.