This is basically layers of sponge cake soaked with liquor and simple syrup with a sweet ricotta filling (sometimes with chopped chocolate or candied fruit or candied rind mixed in) sandwiched between them. It's formed upside down in a plastic-lined cake or pie plate and then flipped out and ringed with a marzipan strip. Some recipes have you ring it before completing the assembling. The traditional garnish is candied whole fruits or fruit-shaped marzipan.
In a saucepan, bring the milk and butter to a boil. Meanwhile, with a whip attachment beat the eggs 5 to 7 minutes until light and very fluffy. Add the sugar and vanilla and mix well, then gradually add the flour. Drizzle in the hot milk, then add baking powder. Pour it into a greased 9-inch cake pan with parchment paper lining the bottom. Bake for 30 to 35 minutes Let cool in the pan on a wire rack then use a blunt knife to go around the edge of the pan to loosen it. Turn it out onto a cutting board and carefully slice it in half horizontally.
To make the filling: Place the ricotta and sugar in a food processor and pulse to blend and make it smooth. Fold in candied fruit, if you are using it. Chill until ready to use.
To make the syrup: In a small saucepan boil the water and sugar together. Let cool then add the orange liqueur.
To assemble the cassata: Butter a 9-inch pie plate and line it with plastic wrap with some hanging over the edge. Place the green marzipan strip around the edge of the pan on the sloping sides, over lapping at where the ends come together. Trim 1 layer of cake to fit into the bottom of the pie pan and place it in. Brush it with half the syrup. Spread the cake layer with ricotta filling. Brush the second layer of cake with the remaining syrup and flip it onto the ricotta filling. Press down lightly to secure it. Cover it with plastic wrap and chill at least 2 hours or overnight.
To serve, uncover the cassata and flip it onto a serving platter. Peel the plastic wrap off the top and dust it heavily with powdered sugar. Decorate with whole candied fruit or fruit shaped marzipan. Serve in wedges.