Step 1

Preheat the oven to 350F/180C. Lightly butter the inside of a 6 1/2 in cast iron skillet (16.5cm frying pan).

Step 2

Whisk together the flour, cocoa powder, salt and cinnamon and set aside.

Step 3

Melt the butter and 1/4 cup chocolate chips together in a heatproof bowl. You can either do this in short bursts of around 20-30 seconds in the microwave, stirring in between, until almost melted, or else in a double broiler. Remove from heat once almost melted and stir to melt the last little bit.

Step 4

Stir the sugar into the chocolate mixture until well combined. Add the vanilla and egg and stir/whisk to mix through. Make sure you do this relatively quickly so the egg doesn't cook as bits of egg (since the mixture is likely still warm).

Step 5

Gradually add the flour mixture and fold in, then fold in the remaining chocolate chips. Once it's all mixed in, pour into the prepared skillet. Spread the mixture to the edges and smooth the top.

Step 6

Bake the brownie in the oven for approximately 18-20 minutes until it is dry on top and gently crisp around the edges. You can test with a skewer in the middle which should come out clean (apart from any chocolate if you catch a chip). Best served warm but you can also serve at room temperature.