Preheat oven to 350°F. Spray 2 regular sized muffin tins with cooking spray.
Whisk together flour, baking soda, and salt, set aside.
Beat butter and sugar on med-high until pale and fluffy (approx. 2-3mins). Reduce speed and add eggs (one at a time), and vanilla. Beat until combined.
Add flour mixture and mix until just combined.
Using a large cookie scoop (3 Tbsp), scoop dough into muffin tins, press down to flatten slightly.
Bake for 10-12 mins or until lightly browned and mostly set.
Remove from oven and immediately use a small jar or container to press firmly down in the center to create a well. Cool in pans for 10 mins, then place on wire rack to cool completely.
Vanilla Custard:
Place 3 cups milk and vanilla bean paste into a medium saucepan. Cook over high heat, stirring often until it just starts to simmer. Remove from heat.
In a medium bowl, whisk remaining 1 cup milk, egg yolks, sugar, and cornstarch.
While whisking vigorously, slowly pour the hot milk into the egg mixture.
Return mixture to saucepan and cook over high heat, whisking constantly, until it thickens and comes to a boil.
Pass through a strainer and into a small bowl. Place plastic wrap directly on top of custard to prevent a skin from forming. Place in refrigerator to cool completely.
Whisk custard to a smooth texture and pipe into cooled cookie cups. Top with fruit if desired.