Preheat oven to 325°F. In a small saucepan whisk together egg yolks, milk, 1 tablespoon sugar, and the cornstarch. Cook and stir over medium heat until mixture is thickened and bubbly. Remove from heat. Transfer mixture to a small bowl; cool for 10 minutes. Whisk in the yogurt.
Divide apple between two 6-ounce ramekins or custard cups. Sprinkle apples with cinnamon and cardamom. Divide yogurt mixture between ramekins. Place ramekins in an 8x8x2-inch baking pan; place on oven rack. Pour enough hot water into the baking pan to come halfway up the sides of the ramekins.
Bake 25 to 30 minutes or until edges are set (centers will appear soft set). Remove ramekins from baking pan; cool on a wire rack for 1 hour. Cover with foil and chill 2 to 24 hours before serving.
Just before serving, let custards stand at room temperature for 20 minutes. Meanwhile, to caramelize sugar, in a small saucepan heat 1 tablespoon sugar over medium-high heat until sugar begins to melt, shaking saucepan occasionally to heat sugar evenly; do not stir. When the sugar starts to melt, reduce heat to low and cook about 2 minutes or until all sugar melts and is golden brown, stirring as needed with a wooden spoon. Quickly drizzle the caramelized sugar over custards. Serve immediately.
We do not recommend using sugar substitute for this recipe.