Oak bolete or Yellow 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.

Cap 6–15 cm, hemispherical in young mushrooms, then cushion-shaped. Color bright yellow to ochre-yellow, with a reddish tinge in the center in mature ones. The surface is matte, slightly sticky in wet weather, and dry and faintly rough in dry weather.

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.

Hymenophore tubular, with a small notch near the stem. Yellow when young, olive-yellow in mature ones. Pores small, round, YELLOW (this sets it apart from its relative Neoboletus luridiformis, which has red pores). They quickly turn blue when pressed.

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.

Stem 6–14 cm tall, 2–4 cm thick, bulbous or slightly club-shaped. Color bright yellow, with fine reddish dots and streaks, but WITHOUT a continuous red net (unlike N. luridiformis). Turns blue when pressed.

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.

Flesh bright yellow, firm in young mushrooms. When cut, it turns blue QUICKLY and STRONGLY — the characteristic reaction of the genus Neoboletus. After 5–10 min it settles to a grayish blue. This is normal and not a sign of toxicity.

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".

Smell pleasant, mushroomy, with no distinctive notes. Taste mild and delicate, with a nutty hint.