Preheat an oven to 350°F (180°C) and line an 8x8 inch metal square baking pan with parchment paper.

Melt butter and chocolate in a large saucepan or microwavable-safe bowl. Heat it on low heat and stir with a wooden spoon or rubber spatula until it's all melted and combined. Alternatively, place it in the microwave and melt it together in 20-second increments, stirring each time.

Once it's all melted, stir through the sugars while the mixture is still hot.

Beat in the eggs, one at a time, until well combined, followed by the salt and vanilla extract.

Sift cocoa powder, cinnamon, and flour into the egg mixture and whisk it well to combine it into a glossy and thick chocolate batter.

In a small microwave-safe bowl, scoop in the cookie butter. Melt it in the microwave for 20-30 seconds to loosen the mixture and make it runny.

Scoop half the brownie batter into the prepared tin and use an offset spatula to smooth it out.

Spoon over the cookie butter spread, reserving 2-3 tablespoons to make some cookie butter swirls at the end. Use the spoon or an offset spatula to smooth it out gently.

Scoop over the remaining brownie batter, adding little dollops all over the cookie butter. Use an offset spatula or butter knife to smooth it out gently. Don't worry if some cookie butter is still peeking through.

Drizzle the remaining cookie butter over the top of the brownies and use a skewer to swirl the cookie butter around to make a pattern.

Bake the brownies for around 28-32 minutes until a knife inserted into the pan comes out reasonably clean with a few moist crumbs sticking to it. No wet batter should be left, but the brownie should still be moist. Take care not to overcook the brownies.

Leave the brownies in the pan to cool down completely on a wire rack before lifting them from the pan and slicing them. For extra clean-cut brownies, chill them in the fridge before slicing.