Inspired by mental__floss's baby cousin.

Knit Donut
Worsted weight yarn, maybe 15g
contrasting yarn for sprinkles
waste yarn
stuffing, such as polyfil or beans
1 stitch marker
Size 6 (4.25mm) double-pointed needles
Tapestry needle
Cast on 20 sts using any temporary cast on method, place marker, join.
Round 1: Knit.
Round 2: Knit 2, make 1 around (30 sts).
Round 3: Knit.
Round 4: Knit 3, make 1 around (40 sts).
Rounds 5-14: Knit.
Round 15: Knit 4, make 1 around (50 sts).
Round 16: Knit.
Round 17: Knit 5, make 1 around (60 sts).
Rounds 18-21: Knit
Round 22: Knit 4, K2 tog around (50 sts).
Round 23: Knit.
Round 24: Knit 3, K2 tog around (40 sts).
Rounds 25-34: Knit
Round 35: Knit 2, K2 tog around (30 sts).
Round 36: Knit.
Round 37: Knit 1, K2 tog around (20 sts).
Round 38: Knit.
The end product:

(To make sprinkles, carry a strand of contrasting yarn and knit a stitch from this yarn every few stitches in rows 6, 8, 10, 12, 14 and 16. For frosting, knit rows 6-16 in a contrasting color.)
Remove the temporary cast on and weave those stitches together with the final row as pictured.

Don't forget to stuff the donut before weaving the final stitches!