In a bowl, beat the unsalted butter, sugar, and salt until combined. Stir in the egg and egg yolks one at a time. Add the yuzu juice and continue to mix. Transfer the bowl onto a double boiler and keep on whisking until the mixture thickens, about 10 minutes.
Remove the bowl from the double boiler and pour the yuzu curd into a clean bowl through a fine sieve to remove any clumps that may have formed. Cover the bowl with plastic wrap and press the wrap onto the surface of the curd so it doesn't form a film. Chill in the refrigerator to thicken.
Crust
Put the butter biscuits in a Ziploc bag and crush them into fine crumbs. Pour the melted butter in the bag and work the butter into the crumbs until combined.
Line a muffin pan with paper liners, and press a heaped tablespoon of the crumb mixture into each paper liner.
Cheesecake
Preheat the oven to 302°F/150°C.
Put the cream cheese and sour cream into a bowl and beat until combined. Once there are no lumps, add the sugar and continue to mix. Add the egg and vanilla extract and stir.
Scoop the batter evenly over the crust and bake for about 20 minutes. The centers of the cheesecakes should still be a little wobbly. Let the cheesecakes cool slightly in the pan before moving them to a wire rack to cool completely.
Chill the cheesecakes in the refrigerator for at least 30 minutes, preferably an hour. Top each cheesecake with as much yuzu curd as you like. Garnish with a candied yuzu peel on top.