Preheat oven to 180C/350F.

Line two baking trays with parchment / baking paper.

Pour 1" / 2.5cm of water into a large saucepan or small pot and bring to a simmer over medium low heat.

Place a bowl over the simmer water and melt the butter and 10oz/300g chocolate chips. Stir until smooth.

While the chocolate is melting, place the eggs, cocoa powder and sugar in a bowl and beat for 3 minutes on high (using an electric beater or stand mixer with the whisk attachment).

Pour 1/3 of the melted chocolate into the egg mixture and mix well, then pour the remaining melted chocolate in and mix again.

Add the vanilla, salt and baking powder and mix until combined.

Add the flour and mix until just combined - do not over mix.

Fold in 1/2 cup dark chocolate chips. See image below for batter consistency.

To make large ones like I did, drop 2 tbsp of mixture onto the baking tray, leaving 2"/5cm between each cookie. To make normal sized ones, use 1 tbsp of mixture per cookie. (Note 1)

Bake large cookies for 15 minutes for very soft or 17 to 20 minutes for firmer cookies. Bake small ones for 10 to 12 minutes for very soft, or 15 minutes for firmer. (Either way it is still moist!)

Allow to rest on the baking tray for 5 to 10 minutes before carefully transferring to a cooling rack.

Place all ingredients except the cream in a stand mixer bowl with the paddle attachment. Beat for 5 minutes on high, or until creamy. Scrape down the sides as you go. Add the cream then beat for a further 2 minutes, or until the frosting is fluffy.

This can also be done using a hand held beater but it will take a few minutes longer.

Spread Peanut Butter frosting on a cookie, then sandwich with another cookie. Repeat with remaining cookies.