Pour <quantity>500<unit>ml</unit></quantity> of buttermilk into the bowl of a stand mixer

Add <quantity>40<unit>g</unit></quantity> of fresh yeast and mix on low speed with the hook attachment until the yeast dissolves

Add <quantity>900<unit>g</unit></quantity> of all-purpose flour, <quantity>100<unit>g</unit></quantity> of sugar, and the seeds from <quantity>1</quantity> vanilla bean. Reserve the pod

Add <quantity>8<unit>g</unit></quantity> of salt and mix on medium speed until a crumbly dough is formed

Add <quantity>50<unit>g</unit></quantity> of butter, diced in small cubes. Mix on medium speed until incorporated

Turn the mixer to medium-high speed. Add <quantity>4</quantity> eggs, one by one, until the dough is shiny and smooth

Preheat the oven to <temperature>50<unit>°C</unit></temperature>

Dust a working surface with flour and put the dough on top. Dust more flour on the dough

Gently fold the dough in on itself until a ball is formed. Dust the dough with more flour as needed

Divide the dough into <quantity>60<unit>g</unit></quantity> pieces

Roll the pieces into tight balls and place them on a sheet of parchment paper, <quantity>5<unit>cm</unit></quantity> apart from each other

Cover with a tea towel and leave in a warm place for about 2h to double in size

Place the vanilla pod in the oven for 1h to dry

Add <quantity>2<unit>cups</unit></quantity> of milk and <quantity>120<unit>g</unit></quantity> of egg yolk to a saucepan

Add <quantity>50<unit>g</unit></quantity> of cornstarch and a mix of <quantity>150<unit>g</unit></quantity> of sugar and the seeds from <quantity>1</quantity> vanilla bean

Turn the heat to medium and whisk continuously until the cornstarch is activated and the mixture is very thick

Pass the custard base through a fine sieve

Place it in the refrigerator until it is slightly chilled

Pour <quantity>½<unit>cup</unit></quantity> of heavy cream into a stand mixer bowl. Whip with a whisk attachment on medium speed until soft peaks are formed

Add the whipped cream to the custard base and mix to incorporate

Season with the zest of <quantity>1</quantity> lemon and mix

Transfer to a piping bag with a tip. The custard filling is ready

Warm <quantity>6<unit>cups</unit></quantity> of vegetable oil in a tall pot to <temperature>165<unit>°C</unit></temperature> over medium-high heat

Blend the dried vanilla pod into a powder

Mix the vanilla powder with <quantity>100<unit>g</unit></quantity> of sugar

Blend <quantity>10<unit>g</unit></quantity> of dried rosebud into a coarse powder

Mix the rose powder with the vanilla sugar and transfer to a wide container

Remove the towel from the proofed dough balls

Cut the parchment paper around the shaped doughnuts

Place the doughnuts in the oil with the parchment paper. Remove the paper after 15s. Fry until both sides of the doughnuts are deep golden brown

Coat each doughnut with the sugar mix

Make a hole on top of each donut with the tip of the piping bag and fill it with the custard to taste. The vanilla & rose doughnut is ready

Enjoy!