Design Eat Repeat

0 Preheat oven to 350℉. Make DIY foil pan according to step-by-step instructions in post above.
2 In a small microwave-safe bowl, melt the unsalted butter ([adjustable]½ cup, i.e. 1 stick[/adjustable]) for 20-30 seconds until JUST melted. 
3 Add in granulated sugar ([adjustable]¼ cup) and light brown sugar (½ cup) and use a spatula to stir until combined (do not use a mixer!). Add in egg (1 large) and vanilla extract (1 ½ teaspoons), then mix. Next, add in baking soda (½ teaspoon), baking powder (¼ teaspoon), and salt (¼ teaspoon) and mix again. Add in all-purpose flour (1 ½ cups[/adjustable]) and mix until all the flour disappears into the dough.
4 Add [adjustable]1/2 cup of chopped chocolate chips and 1/2 cup[/adjustable] of whole chocolate chips into the dough and mix. 
5 Place a piece of parchment on top of the printable heart template. Press cookie dough into heart shape, then wrap with greased foil strip (see instructions in post above). Remove the paper heart template from underneath the parchment paper before you bake.
6 Bake at 350℉ for 16-18 minutes (12-14 minutes if using a real metal pan). Do not overbake or your chocolate chip cookie cake will not be soft.
11 After baking, cool for 5 minutes then remove foil strip. Cool completely before decorating.
7 To decorate: Using a large star piping tip, pipe around the edges then immediately add sprinkles (the frosting will start hardening quick, so add sprinkles right away!).If you're feeling fancy, you can double the frosting recipe to have enough to pipe phrases like "Happy V-Day" on the top. Or use this fool-proof letterboard cake topper kit (these are made from plastic so you can wash and reuse for years!).
To Make Frosting:
10 Beat the softened unsalted butter ([adjustable]¼ cup[/adjustable]) on medium-high speed for 2 minutes until smooth. Turn the mixer to low speed then add the powdered sugar 1/4 cup at a time ([adjustable]1 ¼ cups[/adjustable] total). You may need to stop and scrape down sides. Add vanilla extract ([adjustable]¾ teaspoon[/adjustable]) and a pinch of salt and turn mixer to medium speed. One tablespoon at a time, add milk ([adjustable]1-2 tablespoons[/adjustable] total). Beat on high for 2-3 minutes until creamy and the frosting is at a good consistency to pipe. If it looks too runny, add more powdered sugar. If it looks too stiff, add a tiny bit of milk until you get to the desired consistency.This will make just enough frosting to pipe a border along the edges. If you would like more frosting to cover the top or to write words on top, simply double the frosting recipe!