Melt the chocolate and butter with a pinch of sea salt in a heatproof bowl over a pan of gently simmering water, then remove. Reserving 4 perfect walnut halves for decoration, slice or crumble up the rest. Roll your ice cream into nice round scoops and divide between four teacups. Scatter over the sliced or crumbled walnuts, pour a shot of hot espresso into each cup, stick a walnut proudly on top, then drizzle over the melted chocolate.