Chop dried blueberries into 3–4 pieces and soak in ~½ cup boiling water for ~5 minutes; drain thoroughly.

In a bowl, melt butter and beat in sugar until fluffy. Add sour cream, egg yolks, and vanilla, stirring until smooth.

Stir in flour, baking powder, and salt until mostly combined. Fold in blueberries until no flour streaks remain.

Press dough into 1–2″‑thick disc, wrap well, and refrigerate for 1–2 hours (or up to 24 hrs).

Roll chilled dough to ~1″ thickness on floured surface, cut 3–4″ donuts and 1″ holes. Re‑roll scraps once to maximize yield.

Heat oil (neutral high‑smoke point) in a pot to 375 °F. Fry donuts 2 at a time: 1½ min per side until golden brown. Drain on rack.

Heat milk until steaming, whisk in powdered sugar gradually to form smooth glaze, stir in vanilla.

Dip cooled donuts in glaze both sides, allow excess to drip off, then place on rack to set.