Preheat your oven to 180 C and line the base of a 22cm spring form tine with baking paper.
Place the butter and 3/4 cups of the caster sugar into a bowl and beat with an electric mixer until pale and fluffy. Add the rind and vanilla and beat to combine.
Add the egg yolks, one at a time, beating well after each addition. Add the almond meal and ricotta and beat until smooth.
In a separate bowl, whisk the egg whites until soft peaks form. Gradually add the remaining 1/2 cup sugar and beat until stiff peaks form. Add to the cake batter and carefully fold through, trying to keeps as much air as possible in the mixture.
Spoon into the prepared tin and scatter the sliced almonds over the top. Bake for 1 hour or until firm.
Allow to cool completely in the tin before removing. Dust liberally with icing sugar and serve.
