Oak bolete or Iodine bolete

Cap 6–20 cm, hemispherical in young mushrooms, then flat-convex to cushion-shaped. The color varies: brown, yellow-brown, olive-brown, darker in the center. The surface is dry, slightly sticky in wet weather; in mature mushrooms it may crack finely into soft patches.

The cap of this bolete usually reaches 5 to 15 centimeters across, although larger specimens up to 20 centimeters are sometimes found. When young it is perfectly hemispherical, opening over time to convex-flattened or almost flat, with an even, slightly inrolled margin. Its surface is strikingly matte, true velvet to the touch, and only in old age may become somewhat smoother. The cap's palette ranges from soft light brown and ochre-yellowish to the shade of freshly baked rye bread or a clay riverbank. Notably, the color stays fairly stable and does not change sharply with moisture.

The spore-bearing surface is made of tubes, with a slight notch near the stem. The color is rich yellow (a buttery yellow — hence the genus name "butyri"). The pores are small and round. When pressed they may turn slightly blue, especially in mature mushrooms.

Under the cap hides a tubular hymenophore that forms a free or slightly notched zone right at the stem. The tubes and pores are very small and tightly packed. In very young mushrooms they are a soft pale lemon, which shifts to a rich bright yellow as the spores mature, and later to olive-yellowish. This hymenophore has one extremely important diagnostic feature: unlike many related species, it does not change color when pressed or damaged and does not turn blue at all, keeping its original sunny shade unchanged.

Stem 5–15 cm tall, 2–6 cm thick. Thicker near the top; toward the base it sometimes tapers or has a cone-shaped "appendage" (hence appendiculatus). Yellow in color, with a barely visible fine net in the upper part, sometimes with reddish-brown spots at the base.

The stem of the iodine bolete grows 5 to 15 centimeters tall and 2 to 5 centimeters thick. It is often cylindrical, but when young it can be noticeably widened toward the base, like a sturdy club. In the hand it feels very firm and massive. Its surface is pale yellow or lemon-colored, shading near the base into reddish-brown or even reddish tones. Unlike porcini, this stem has no characteristic net; instead it is slightly rough-fibrous and covered with tiny, barely visible dots. There is no ring or volva at all.

The flesh is buttery yellow, very firm in young mushrooms, softer in mature ones. When cut it may slowly turn blue, especially in the tubes and the upper part of the stem. At the stem base it sometimes has a pink or reddish-brown tint.

The flesh of this mushroom is remarkably meaty, thick and firm to the touch. In the cap it stays whitish or pale yellow, while in the stem it turns distinctly bright yellow. The most important identification marker is its reaction to air: at a break or cut the flesh stays stable and does not change color; only very rarely, when badly damaged, may it take on a barely noticeable pinkish tone. The taste is pleasant, slightly sweetish, without any bitterness, which makes it an excellent ingredient after cooking.

The smell is pleasant, mushroomy-nutty, with a faint note of fresh forest berries. The taste is mild and earthy, with no bitterness — some sources describe it as "buttery nuttiness".

The smell is the most interesting and most controversial feature of this species. A freshly cut mushroom, especially at the very base of the stem, gives off a distinct medicinal aroma most often compared to iodine or carbolic acid. This unusual "hospital" smell may seem off-putting when you're picking in the forest and is the reason many foragers have doubts. But don't worry: this chemical aroma is purely volatile. When heated, boiled or dried, the smell disappears completely, leaving only a classic, warm mushroom aroma behind.