In a large bowl, whisk together warm water, milk, sour cream, oil, egg, and salt until blended.

Using a wooden spoon, add 1 cup flour and mix until incorporated. In ¼ cup increments, add ¾ cup of flour to the dough and just enough so that the dough no longer sticks to the sides of the bowl. (You will use the remaining ¼ cup flour when you roll out the dough.)

Knead 2–3 minutes or until the dough is smooth and elastic. Cover with plastic wrap and let rest for 30 minutes.

Meanwhile, make blueberry jam by combining blueberries, lemon juice, and sugar in a small saucepan on medium heat. Allow blueberries to burst and simmer long enough so that mixture is slightly thicker than a normal jam would be. Let cool.

Combine blueberry jam with mascarpone, cream cheese, vanilla bean paste, and salt. Split dough into 4 equal pieces and begin rolling out the first ball on a floured surface. Dough should be thin and slightly translucent when held up. Cut into rounds using the top of a drinking glass or cookie cutter. (Each quarter should make 8 rounds.)

Spoon a dollop (slightly less than a tablespoon) of blueberry cheese mixture onto the middle of each round. Be careful not to overfill.

Fold rounds in half and pinch edges closed. I pinch and twist to make a spiral edge, but no need to worry about looks here—just make sure filling doesn’t leak.

Bring a pot of salted water to a boil. Make sure there is plenty of water so the pierogies don’t stick to each other.

Add pierogies to boiling water but don’t overcrowd. Once they rise to top of surface, cook for an additional minute. Remove from water.

Heat a pad of butter in a frying pan with a generous pinch of sea salt and some sugar. Add pierogies to pan and fry until crispy and golden brown. Serve with plain mascarpone and top with flaky sea salt.