Amber Black's Chocolate Banana Bread Squares

Dense yet moist, and gorgeously golden within.

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   1. Have ready a 22cm square cake tin or equivalent rectangular tin. Lightly butter the tin and preheat the oven to 190C/170C fan/375F/gas 5.
   2. Chop the chocolate and combine with the remaining ingredients for the chocolate mixture. Set aside.
   3. For the banana mixture, first peel the bananas and mash well. Sieve together the flour, bicarbonate of soda and salt.
   4. In a bowl, using an electric hand whisk, beat the butter and sugar together until pale and fluffy. Beat in the eggs one at a time, followed by the bananas, yoghurt and vanilla, at which point the mixture will appear curdled. Gently beat in the sifted flour until just combined. Do not over-mix.
   5. Transfer half the banana mixture evenly to the buttered tin. Sprinkle over half the chocolate mixture. Repeat with the remaining banana and chocolate mixtures.Bake until the top is golden, about 35-40 minutes. Leave to cool in the tin for 25 minutes, then turn out on to a rack to cool completely.
   6. To finish, gently heat together the chocolate squiggle ingredients in a small pan, stirring until melted and smooth.
   7. Trickle over the top of the banana bread to decorate (any left over is good with ice cream). Serve cut into squares.