The recipe is one of Dan Lepard's, see here, that I've never managed to get right. At the end of my last attempt, I thought that perhaps using the chocolate filling on top of a layer of dulce de leche might work better than trying to 'gently swirl' the caramel mixture Dan uses into the chocolate. So this time, I baked the pastry case blind, and gently spread about 4 heaped tablespoons of dulce de leche onto the hot pastry, before pouring in the chocolate mixture.
This proved to be the right thing to do - but my mistake was trying to gild the lily with a swirl of pure dulce de leche as a topping. This caramelised during baking and sank into the chocolate mixture, leaving a swirly trench on top of the tart. However, the fudgy malted chocolate on top of a layer of dulce de leche worked really well and tasted rich and decadent. Definitely the way to go forward.