A classic cheesecake.
Stir buttermilk into whipping cream and pour into a plastic container like a juice pitcher and cover with plastic wrap. Place pitcher in a larger container, and pour hot tap water around the pitcher, filling up to the height of the cream. Place container in a warm, draught-free spot (I find on top of the fridge is best) and let sit for 24 to 48 hours (it ripens faster in summer than winter).
To determine if set, jiggle the pitcher slightly – the cream should be not shift and should have sweet-lemony smell. Chill completely. Spoon off crème fraiche, being certain not to stir in the liquid whey, which will settle at the bottom of the pitcher.
For crust preheat oven to 350°F. Beat butter and brown sugar until light and fluffy. Beat in egg until smooth. Stir in flour and mix until dough comes together. Divide dough in half, wrap one half and chill, and spread the other half of the dough onto the bottom of an ungreased 10-inch springform pan (it will cover the bottom of the pan very thinly). Bake for 10 minutes and allow to cool. Once cooled, press the other half of the dough onto of the first layer. Don’t worry if dough seems uneven – it will all even out in the end. Do not bake second layer.
For filling, increase oven temperature to 450°F. Beat cream cheese until very fluffy and smooth, scraping down the sides of the bowl often. Gradually add sugar while beating, again, scraping down the sides of the pan often. Beat in flour. Add eggs one at a time, waiting until egg is fully incorporated before adding next. Beat in vanilla and ½ cup crème fraiche or sour cream. Scrape batter onto crust base and bake for 10 minutes. Reduce oven temperature to 300°F and bake for 30 minutes.
While cheesecake is baking, prepare topping, Stir crème fraiche or sour cream, sugar and vanilla together until sugar dissolves. Remove cheesecake from oven carefully and gently spread crème fraiche topping over cheesecake, taking care not to tear the surface of the cheesecake. Return to oven and bake 15 minutes. Cool cheesecake for 30 minutes, then run a spatula of knife gently around the edge of the cheesecake, to loosen (this will prevent any cracking). Cool cheesecake to room temperature and chill overnight.
To serve, remove springform pan ring and slice with a hot, dry knife. Serve with fresh berries.