Preheat the oven to 190°C/375°F and line a 20cm square baking tin with parchment paper. Melt the butter in a pan over a low heat or in the microwave. Add the chocolate and then remove it from the heat and leave the chocolate to melt.
Put the eggs, sugar and vanilla in a bowl and using a hand-held electric mixer whisk it up until light and frothy. Add the flour and gently whisk this all together along with the salt. Then pour in the chocolate and butter mixture along with the pecan nuts, gently folding it all together until well combined. Pour all of this into the prepared tin.
Pop it in the oven for 22-25 minutes until the top is crusty but there is still a nice wobble in the middle. Once baked, remove from the oven and leave to cool down a little before cutting it into eight even-sized squares, which will crack as you cut them. Serve warm on plates with a dollop of crème fraiche and top with fresh strawberries if you like.