Brownies with Cookie Buttercream

Ingredients

• 13 tbsp butter, unsalted
• 6 1/2 oz 70% dark chocolate (185g), finely chopped
• 3 eggs, straight from the fridge
• 2 tsp vanilla
• ½ tsp instant espresso powder, optional
• 1 cup less 1 TBSP white sugar
• ¼ cup packed light brown sugar
• ½ tsp coarse kosher salt (half as much if using fine table salt)
• ½ cup plus 1 TBSP all purpose flour
• ½ cup cocoa powder
• 2 oz white chocolate chopped
• ¼ cup milk chocolate chips
• additional shortcake cookies for garnish
• 200 g shortcake cookies (lotus biscoff ideally, but replace with regular ones if cannot locate) - that's around 2/3 of a standard box
• 11 tbsp butter, unsalted at room temp
• 1 ¾ cups icing sugar
• 1 tsp vanilla
• pinch of kosher salt
• 3 tbsp heavy cream, plus more if needed