Heat the oven to 180°C/160°C fan/gas 4. Grease a 12-cup muffin tin.
Melt the butter with the chocolate & the coffee over a saucepan of barely simmering water, or 1-2 minutes in the microwave in short bursts, stirring between each. Leave to cool for 2-3 minutes.
In a large mixing bowl, using an electric mixer, whisk the eggs, sugar and a pinch of salt until pale and fluffy.
Fold in the cooled chocolate and vanilla, then fold in the flour and cocoa. Gently fold everything together with a pinch of flaky sea salt until combined - stop as soon as there's no streaks of flour.
Fill each muffin case just over half-way. Top each with a heaped teaspoon of peanut butter.
Cover with the remaining cake mixture.
Bake for 12-20 minutes until the brownie-cake has pulled and just cracking on top. They should still have a slight wobble in the centre.
Leave to cool in the tin for 5 minutes, then turn out onto a wire rack to cool completely.