Preheat oven to 350°F (175°C). Grease and line an 8-inch square baking tin.

Melt the butter and chocolate together in a heatproof bowl over simmering water.

In a separate bowl, beat eggs and sugar until creamy and light. Add vanilla extract.

Gradually fold in the melted chocolate mixture.

Sift in the flour and salt, mixing until just combined.

Pour half the batter into the prepared tin. Place the halved truffles evenly and then cover with the remaining batter.

Bake for 25-30 minutes or until set but still slightly gooey in the middle.

Let it cool completely before slicing.