150 g unsalted butter, plus extra for greasing
300 g light muscovado sugar
75 g cocoa powder
150 g plain flour
1 tsp bicarbonate of soda
1/4 tsp flaky sea salt (plus extra 1 tsp for garnish)
4 eggs
1 tsp vanilla extract
150 g milk chocolate chips
Juice and zest of 2 limes Extra zest of 1 lime for garnish (optional)
Cut into 12 pieces, and top with some extra lime zest if you like.