From https://cookiesandcups.com/texas-sheet-cake-cookies/

INGREDIENTS

Cookies
1/2 cup butter, room temperature
1/3 cup granulated sugar
1 egg
1 tsp vanilla
1 tsp baking powder
1/2 tsp salt
1 1/3 cup flour
1/2 cup semi-sweet chocolate chips, melted

Icing
1/2 cup butter
2 Tbsp cocoa powder
3 Tbsp milk
2 1/2 cups powdered sugar

INSTRUCTIONS

Preheat oven to 350°
Line baking sheet with parchment
In bowl of stand mixer beat butter and sugar together until light, scraping sides frequently.
Add in egg and vanilla and continue mixing until incorporated.
Mix in baking powder and salt.
Turn mixer to low and slowly add in flour. Dough will be thick.
In a microwave safe bowl heat chocolate chips on high in 30 second increments until melted, stirring after every 30 seconds.
Mix melted chocolate directly into cookie dough until evenly mixed.
Drop dough by a measured tablespoon sized mounds onto baking sheet.
Bake for 7-8 minutes until cookies just appear set. They will still be very soft on the inside. DO NOT overbake!!
Transfer to a wire rack to cool.

Icing
In a medium saucepan combine butter, cocoa powder and milk over medium heat, whisking until melted together.
Remove from heat and whisk in powdered sugar.
Pour icing over cookies and allow icing to set before serving.
Yes, the cookies can basically just be drenched with icing. Let them set up and them move them with a spatula to whatever plate you’ll be serving them from, or whatever container you’ll be storing the in.