Lemon curd: Mix the lemon zest, juice, sugar and eggs in a small saucepan. Add the butter and heat on a medium heat while stirring until traces of the whisk remain visible and the first bubbles appear on the surface. Remove from heat and strain through a sieve into a wide bowl. Cover with cling film (place the film directly on the surface) and let it cool completely. Ground: Mix all ingredients for the base, press onto the base of a springform pan (26 cm), forming an edge about 2.5 cm high. Bake the springform pan on the middle rack at 175 ° in the hot oven for 10 minutes. Reduce the oven temperature to 150 °. Mix the cream cheese and sugar in a bowl with the whisk of the mixer for about 1 - 2 minutes at medium speed until creamy. Now stir in the eggs one after the other on a low heat. Stir in the sour cream and vanilla extract. Pour 2/3 of the cream cheese mixture onto the bottom, then pour half of the lemon curd on top. Mix in a spiral with a small knife. Repeat with the remaining filling and lemon curd. Bake the cake for about 45 - 60 minutes, until it is firm about 4 cm from the edge, but still appears soft in the middle. The filling then looks very soft, but solidifies when it cools down. Take out of the oven and immediately remove the edge from the tin with a knife. Let cool down for about 2 hours, then leave to cool in the refrigerator for at least 4 hours without a cover.