Texas Sheet Cake Cookies

Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.
Melt the chocolate chips in a microwave-safe bowl, microwaving in 30-second bursts and stirring between each until smooth. Let cool slightly.
Whisk together flour, cocoa powder, baking powder, baking soda, and salt in a medium bowl.
In a large bowl, beat softened butter, granulated sugar, and brown sugar until light and fluffy.
Beat in the egg, egg yolk, and vanilla extract until smooth. Pour in the cooled melted chocolate and mix to combine.
Using a spatula, stir in the dry ingredients until just combined. If dough is too sticky, chill for 30 minutes.
Scoop rounded tablespoons of dough onto prepared baking sheets, spacing them about 2 inches apart.
Bake 8–10 minutes, just until edges are set and centers are still slightly soft. Cool on sheets for 5 minutes, then transfer to wire racks.
For the frosting: In a small saucepan, melt butter over medium heat. Stir in cocoa powder and buttermilk until smooth. Remove from heat and whisk in powdered sugar and vanilla extract until glossy and spreadable.
Spoon warm frosting over each cookie, spreading to the edges. If desired, sprinkle with chopped nuts or flaky salt before frosting sets. Let frosting set up before serving or storing.