1 tbsp. unsalted butter, at room temperature, cut into pieces
1/2 tsp. kosher salt
1 1/2 c. all-purpose flour, spooned and leveled, plus more for work surface
1/2 tsp. lemon zest
2 qt. canola oil, plus more for bowl and baking sheet
8 oz. blueberries (2 cups)
6 tbsp. loose rooibos tea
2 c. confectioners' sugar
Heat milk in a small glass measuring cup in microwave on high for 30 seconds or until just warm to the touch (between 100°F and 110°F). Stir in yeast and granulated sugar. Set aside until frothy, 4 to 6 minutes (if mixture does not froth, discard and start again). Transfer to a bowl; add egg and whisk to combine. Whisk in butter and salt (don’t worry if some lumps of butter remain).
Stir in flour and lemon zest until a dough forms. Beat dough with an electric mixer fitted with dough hook on low speed, 5 minutes (dough will be sticky). Lightly oil a bowl. Transfer dough to prepared bowl and cover with a towel. Let rise, at room temperature, until doubled in size, 1 to 1 1/2 hours.
Lightly oil a large baking sheet. Turn dough out onto a lightly floured work surface; lightly dust dough. Roll into an 18-by-1 1/4-inch rope. Cut into 1-inch pieces and roll into balls. Transfer to prepared baking sheet and cover with a towel. Let rise, at room temperature, until puffed, 25 to 30 minutes.
Place a wire rack on a baking sheet. Heat oil in a large saucepan to 350°F to 360°F. Cook donuts, 3 or 4 at a time, until golden brown, 1 to 2 minutes per side. Transfer to wire rack using a slotted spoon; let cool.
Cook blueberries, rooibos, and 1/2 cup water in a small saucepan, pressing berries with a potato masher to mash, until syrupy, 8 to 10 minutes. Strain through a wire-mesh strainer lined with cheesecloth; discard tea. Add confectioner’s sugar and whisk until smooth. Dip tops of donut holes in glaze.