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1
In a food processor, blend the almonds and pistachio together until you have a very fine crumb. When you press it between your fingers it should almost hold together, like a dough. Set aside.
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2
Place the sugar and water together in a small saucepan over a low-medium heat and bring to the boil. Let it boil for about 10 to 15 minutes or until when you dip a spoon in it, rather than drip, the syrup should thread. Remove from the heat and add the nuts to combine with a wooden spoon. Turn out onto a clean, smooth surface to cool.
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3
When cool enough to handle but still warm, form into a ball and roll out with a rolling pin to a thickness of 1/3 inch (8 millimeters). If the marzipan sticks at all, dust with powdered sugar. Cut the marzipan into strips the height of your pie tin then cut those strips into pieces approximately an inch wide on one side and 1 1/2 inches on the other so that you have trapezoid shapes. No need to be super precise, eyeballing is fine. Set aside the marzipan pieces until needed.
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4
Slice the sponge cake (here is my recipe for a simple, fluffy sponge, which also happens to be gluten free: http://food52.com/recipes/27584-gluten-free-sponge-cake) into discs about 1cm thick (a little over 1/3 inch). You want 2-3 good discs from the sponge. One disc will be the base (later, it will become the top) of the cake. With another, cut strips then trapezoids the same way you did with the pistachio marzipan. With the final disc, set aside for the top (later to become the bottom) of the cake. Some also use simply the sponge leftovers for this part, breaking them into crumbs to sprinkle over the cake in a layer. If your sponge is not the same diameter as the pie tin, you can also slice the sponge into fingers and arrange them (much like you would tiramisu) in a layer across the bottom.
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5
For the ricotta filling, start with a fairly stiff ricotta that can stand on its own if you were to tip it out of its container. If your ricotta is quite soft, spoon it into a colander lined with a damp tea towel or a few layers of muslin or cheesecloth set over a bowl. Leave it overnight for the excess liquid to drip out. Combine firm ricotta and caster sugar together until smooth and creamy. Add the dark chocolate, roughly chopped into fine pieces. Set aside.
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6
To assemble the cassata, use a pie dish that measures about 10 inches (25cm diameter) along the bottom. Place a disc of sponge cake on the bottom. Assemble the marzipan and sponge trapezoids along the sides of the tin, alternating to achieve a striped look. Don't worry too much about any little gaps.
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7
Fill the sponge with the ricotta mixture, using a spatula to cover all the way to the sides and smoothing the top. Place a sponge disc (or a sponge crumb layer) on the top and cover with plastic cling wrap. Place a small flat plate that fits just inside the width of the pie tin on top and weigh the cassata down with a couple of jars of jam in order to keep the cassata pressed. Let rest in the fridge for at least 2 to 3 hours, or even better, overnight.
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8
Remove the plastic wrap and place a flat serving plate over the top of the cassata, then carefully upturn the plate and remove the pie tin (it will come out very easily). Prepare a sugar glaze by combining the powdered sugar and water in a small saucepan and heating to a simmer. It should be thick but still runny -- add water as needed, a tablespoon at a time, until you get the right consistency.
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9
It helps to have 4 hands for this next part. Tip the hot sugar glaze out onto the cassata and very quickly smooth over to cover the entire cassata, top and sides (it cools quickly and as it cools it hardens so it is easier to do this bit by bit).
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10
Decorate with artisan candied fruit, cut into slices or pieces or left whole. Serve sliced into wedges.
