In a medium bowl, whisk together flour, baking powder, baking soda, salt, 0.5 teaspoon cinnamon, and a pinch of nutmeg. Set aside.

In a large bowl, beat the eggs and 0.5 cup sugar until pale. Stir in sour cream and vanilla until smooth.

Add the dry ingredients to the wet mixture. Fold gently until just combined. The dough should be soft and slightly sticky. Do not overmix.

Lightly flour a work surface. Turn dough out and pat to about ¾-inch thickness. If dough is too sticky to handle, chill for 10–15 minutes.

Cut donuts with a 3-inch round cutter and a 1-inch cutter for the hole. If you don’t have cutters, use a glass and a bottle cap. Re-roll scraps once.

Pour oil into a heavy skillet or Dutch oven to a depth of about 1½–2 inches. Heat oil to 350°F (175°C). Use a candy or deep-fry thermometer for accuracy.

Fry donuts in small batches. Cook for 60–90 seconds per side, until golden brown. Don’t overcrowd the pan — they should float freely.

Transfer to a cooling rack set over paper towels to drain briefly (20–30 seconds).

While still warm, toss each donut in the cinnamon-sugar mixture (combine ⅔ cup sugar with 2 teaspoons cinnamon in a shallow bowl). Ensure even coating.

Let cool slightly before serving. Enjoy warm.