Start with making the filling.  The filling needs to firm for at least one hour, so I suggest making this first and then baking the cupcakes.  To make the cookie dough filling, combine the butter and brown sugar in the bowl of a stand mixer and beat until light and fluffy.  Add the flour, sweetened condensed milk, and vanilla and mix until smooth.  Mix in the chocolate chips.  

Remove the bowl from the stand mixer and cover with plastic wrap.  Place the bowl in the refrigerator and let chill for at least 1 hour, until firm.

To make the chocolate chip cupcakes, preheat the oven to 350 degrees.  Place baking cups in 2 cupcake pans.  Set aside until needed.

In the bowl of a stand mixer, beat together the butter and brown sugar until light and fluffy.  Add the eggs, one at a time, beating well after each addition.  

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.  

Alternating with the milk, add the flour mixture in 3 additions, starting and ending with the flour mixture.  Mix in the vanilla.  Remove the bowl from the mixer and, with a spatula, fold in the chocolate chips

Using a ice cream scoop, fill the baking cups 2/3 of the way up.  Bake the cupcakes until a toothpick inserted into the center of a few of them comes out clean, about 20 minutes.  Let the cupcakes cool in the pan for about 10 minutes, and then remove from the pan and place on a cooling rack to cool completely.  

While the cupcakes are cooling, make the cookie dough buttercream.  In the bowl of a stand mixer, beat the butter and brown sugar until light and creamy.  Add the confectioners sugar and mix until fully incorporated.  Mix in the flour and salt.  Add the milk and vanilla and beat until smooth and creamy.

To fill the cupcakes, cut/poke a hole in the middle of each cupcake.  I use an apple corer; I think it’s the easiest way that creates the best results.  If you do not own a apple corer, you can use a small knife to cut a hole in the middle of the cupcake.  If the cookie dough filling is hard enough, place a small piece of cookie dough into each hole.  If it is still a little bit loose, place the filling in a piping bag and pipe small amounts in the center of each hole.

To frost the cupcakes, place the frosting in a piping bag fitted with your favorite tip.  Pipe the frosting in a circular motion onto each cupcake.  If desired, sprinkle with chocolate sprinkles or mini chocolate chips.