To Make the Cookie Cups:
Preheat your oven to 350°F (175°C). Spray two regular-sized muffin tins with cooking spray.
In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
In a large mixing bowl, beat the butter and sugar on medium-high speed until pale and fluffy, about 2-3 minutes.
Reduce speed and add the eggs, one at a time, beating well after each addition. Add the vanilla extract and mix until combined.
Gradually add the flour mixture to the wet ingredients, mixing until just combined.
Using a large cookie scoop (about 3 tablespoons), scoop the dough into the muffin tins and press down slightly to flatten.
Bake for 10-12 minutes, or until the edges are lightly browned and the centers are mostly set.
Remove from the oven and immediately use a small jar or container to press firmly down in the center of each cookie to create a well.
Let the cookie cups cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.
To Make the Vanilla Custard:
In a medium saucepan, combine 3 cups of milk and the vanilla bean paste. Cook over medium-high heat, stirring frequently until the milk just begins to simmer. Remove from heat.
In a medium bowl, whisk together the remaining 1 cup of milk, egg yolks, sugar, and cornstarch until smooth.
While whisking constantly, slowly pour the hot milk into the egg mixture to temper the eggs.
Return the mixture to the saucepan and cook over medium-high heat, whisking continuously, until it thickens and comes to a boil.
Pass the custard through a strainer into a bowl to remove any lumps. Cover the custard with plastic wrap, pressing the wrap directly onto the surface to prevent a skin from forming. Refrigerate until completely cooled.
Once the custard is cooled, whisk it until smooth and pipe it into the cooled cookie cups.
Top with fresh fruit if desired. Store the cookie cups in the refrigerator and serve within two days.