In a small bowl, combine lukewarm milk, & 2 Tbsp of the sugar; stir until sugar is dissolved. Add in the yeast & allow to sit until frothy.
In a large bowl, combine mashed potatoes, eggs, salt & butter. When yeast mixture is proofed, add to potato mixture, combining well.
In another bowl, whisk together flour & remaining sugar. Combine with wet mixture until dough forms a ball. Knead on a work surface for about 10 minutes then place in a greased bowl.
Cover with plastic wrap & a towel. Allow to rise in a draft-free place for about 1 1/2 hours or until doubled in size. Lightly butter a 12-hole doughnut pan; set aside.
Punch down the dough & cut into 12 evenly sized pieces. Roll each piece into a strip long enough to fit around each doughnut hole mold. Lay them in the molds & pinch the ends together so the dough rounds are more or less even.
Cover the tray with plastic wrap & a towel & allow to rise for about 30 minutes.
Preheat oven to 350 F.
Bake doughnuts for about 20-25 minutes. The bottom should only be slightly browned while the top is still pale as they will be a bit chewier then.
Blackberry Glaze
While doughnuts are baking, place blackberries in a food processor & puree ; strain. Place in a small bowl & add lemon juice, vanilla & sifted powdered sugar. Combine until fully incorporated & no lumps remain.
Glazing
When baked doughnuts a still slightly warm, drizzle glaze over them & allow glazed doughnuts to set about 20 minutes before serving.
Recipe Notes
I wanted to give my little doughnuts a bit of a fancier look today so I baked them in mini Bundt pans. Same great flavor wearing a new look!