Preheat oven to 170°C fan/190°C/gas mark 5. Butter the bottom and sides of a 23cm spring form tin. Line the sides of the tin with non-stick baking parchment.
Pulse the digestive biscuits and sugar in a food processor until fine crumbs. Add the butter and pulse until mixed. Press this evenly into bottom of prepared tin and bake on a lower oven shelf for about 15 mins until golden brown and set. Transfer to wire rack to cool for 10 mins.
Using an electric food mixer (not food processor) or electric hand whisk, beat the Quark, butter and sugar until smooth. Beat in the eggs and yolks one at a time until fully incorporated, then the lemon zest and vanilla extract.
Take a large piece of thick kitchen foil and place the lined tin in the centre and wrap foil tightly around bottom and sides of pan (this will prevent any water leaking into biscuit base). Set in a roasting pan, pour the cheesecake filling into the tin, and smooth the top
Pour boiling water into roasting pan to come halfway up the sides of the cheesecake and carefully transfer to oven. Bake for 1 hour until top of cheesecake is golden brown, the edges are set.