Traditionally known as a poor substitute for porcini, personally I find them to be every bit as good and also not nearly as often infested with maggots as porcini are.
Cap
A chestnut to dark brown cap, which is almost spherical in young specimens before broadening and flattening out to a diameter of up to 10 cm. The cap has an acute margin and a velvety surface when young, being slightly sticky when wet or old.
Gills
On the cap undersurface, the pores are initially cream to pale yellow, but become greenish yellow or olive with age. They stain dull blue to bluish-grey when bruised or cut, and are easily removed from the flesh. It has olive to olive-brown spores.
Stem
The stipe is 4–9 cm long by 1–2 cm thick, and is similar in colour to the cap but paler, and sometimes with a rose-coloured tinge. Its surface has faint longitudinal ridges, a fine powdering, and fine reticulations (a net-like pattern of ridges) at the apex.
Flesh
The flesh is mostly whitish or yellowish in some places; underneath the cap cuticle, it is brownish-pink or reddish brown. Initially firm, it begins to soften under the cap in older mushrooms.
Leaves
Flowers
Seeds
Stem
Fruit
Taste
The smell is, would you believe, mushroomy! The taste is slightly less strong than porcini, but barely noticeably so.
Frequency & law
Very common in it's habitat and not protected.
The nuanced bit
As mentioned, often seen as a poor cousin of porcini but I think they are easily as good.
Possible confusion
For beginners, maybe porcini (Boletus edulis), but porcini have much chunkier stipes and do not bruise blue.
Habitat
The bay bolete is common in coniferous and less commonly mixed woodlands in Europe, from the British Isles.
How to harvest
Fruit bodies appear singly or scattered on the ground, or on decaying tree stumps, and can be well hidden by pine needles and ferns. Fruiting tends to peak three or four days after rain during warm weather.
Cooking tips
Other uses
Folklore