Begin by making the pastry. Place the butter and flour in a food processor and pulse until the mixture resembles fine breadcrumbs. Add the sour cream and pulse until the mixture starts to come together and form a ball. Wrap the dough in plastic wrap and refrigerate for 30 minutes.
For the candied cumquats place the orange juice and castor sugar in a large saucepan and stir to dissolve the sugar. Bring to a boil and add the cumquats. Return to a boil and simmer until the syrup is thick and the cumquats have collapsed and appear slightly translucent. Remove from the heat and allow to cool.
Preheat the oven to 200 celsius (390 Fahrenheit) and roll out the sour cream pastry until approximately 3mm thick. Line a 22cm loose bottomed tart tin with the pastry and trim the excess. Refrigerate for 30 minutes before lining with foil, and filling with dried beans or pastry weights. Bake it for 10 to 15 minutes or until the pastry starts to dry. Remove the foil and beans and return the pastry case to the oven for a further 10 minutes to ensure the pastry is crisp. Place on a wire rack to cool.
Reduce the oven temperature to 175 celsius (350 Fahrenheit) and place the ground almonds, cinnamon and cumquat zest in a large bow. Stir to combine before adding the cumquat syrup and the butter. Pat the almond mixture into the tart base and bake the tart for 30 minutes, or until golden. Place on a wire rack and cool completely.
To make the chocolate glaze, bring a saucepan of water to a boil and turn the heat off. Place the chocolate, cream, butter and syrup in a bowl and place over the saucepan and stir until melted and smooth. Poor over the top of the tart and spread evenly with an offset spatula.