Chocolate Brownie Sandwich with Peanut Butter Filling

Preheat the oven to 350 degrees F (175 degrees C). Line 2 baking trays with greaseproof paper.
Place the butter and chocolate chips in a microwave safe bowl. Microwave in 15 second intervals, stirring in-between, until the chocolate is fully melted and smooth. Alternatively you can melt the chocolate in a bowl over a pan of hot water - take care that the bowl doesn't touch the water or the chocolate will seize.
Using a hand-held or stand mixer, mix the eggs and sugars for approximately 3 minutes on high-speed until they're light and creamy.
Put the mixer on low and slowly begin to add the chocolate and butter mixture.
Add the flour, cocoa powder, and salt and mix until the dough is smooth.
Using a standard size cookie scooper or ice cream scoop, scoop the dough onto the tray. Spread the cookies 2 inches apart. Bake for approximately 10 minutes or until the cookies have cracked. The centre of the cookie should appear gooey. Allow the cookie to fully cool while making the peanut butter filling.
For the peanut butter filling, combine the peanut butter and softened butter using a hand-held or standing mixer. Beat until creamy and then add the icing sugar, cream, vanilla extract, and salt until fluffy.
Place the peanut butter filling into a piping bag and pipe the filling onto the back side of the cookie. Close the cookie sandwich with another cookie. Repeat the process with the remaining cookies and serve.