Measure out ¼ cup of the sweetened condensed milk and place in a small microwave safe bowl with the chocolate chips. Set aside.
In a medium-sized bowl, stir together peanut butter, milk, and vanilla extract until smooth.
In a separate bowl (I usually use my KitchenAid with the whisk attachment, but you may also use a handmixer with a whisk attachment) beat the heavy cream until stiff peaks form.
Add the peanut butter mixture to the heavy cream and fold in until completely combined.
Pour into a 9x5 breadpan and prepare the chocolate swirl:
Microwave your reserved condensed milk and chocolate chips at 15 second increments (stirring between) until chocolate is completely melted.
Drizzle chocolate across the ice cream mixture, and use a knife to swirl the chocolate throughout.
Cover the breadpan with cling wrap and refrigerate for at least 6 hours (preferably overnight).
Peanut Butter Cookies
Preheat oven to 350F
In a stand mixer, cream together shortening and peanut butter.
Add brown sugar and beat until light and fluffy.
Add egg, vanilla, and milk and stir until combined.
In a separate bowl, combine flour, cornstarch, baking soda, and salt.
Gradually add to peanut butter mixture until completely combined.
Measure out ¼ cup sugar in a small dish and set aside.
Roll dough into 1 ½ Tbsp-sized balls, roll in the sugar, and place on a parchment paper-lined cookie sheet.
Bake on 350F for 8 minutes. Once you remove cookies from the oven immediately gently press down with the tines of a fork to flatten.
Allow to cool on cookie sheet for 5 minutes and then move to cooling rack to cool completely.
Assembly & Chocolate*
If using chocolate** melt it according to instructions on packet and allow it to cool for several minutes.
Prepare a cookie sheet by lining with wax paper. Set aside.
Remove ice cream from freezer and scoop about ¼ cup (for big sandwiches) on top the bottom of one cookie, and then sandwich with another. Quickly dip one half in chocolate, place on cookie sheet, sprinkle with toffee pieces (if using) and immediately return to freezer. Allow cookies to chill for about 3-5 minutes** before enjoying.
Notes
*If you follow these steps and the chocolate isn't cooled enough or if the ice cream is not cold enough, this can make for a messy process. Just be prepared to get your hands dirty when enjoying these sandwiches (it's worth it!). ALTERNATIVELY, you can just dip each of your cookies ⅓-1/2 of the way in chocolate, sprinkle with toffee pieces (if using) allow them to harden at room temperature and then sandwich them around the ice cream. I included the steps to make them look like the pictures in the recipe, but this is an easier and less messy way to make your sandwiches. **Don't make the sandwiches until you are just about ready to serve them. If the cookies stay in the freezer too long they will harden.