Flour or semolina scorches underneath the base, tasting bitter and burnt

Cosmetic

Almost always excess loose dusting flour or semolina left under the dough and exposed to a very hot baking surface.

Ask yourself

How much loose flour or semolina was used to dust the peel and dough?
Was any excess dusting flour brushed off before launching?
How hot was the baking surface?

Likely causes

#1
Excess loose flour or semolina left under the dough, well beyond what's needed to prevent sticking
Strongly supported
Any dusting flour or semolina not absorbed into the dough sits directly against the hottest part of the bake and chars quickly at high stone or steel temperatures, leaving a bitter black residue.
#2
Very high baking surface temperature, common with a well-preheated steel or stone
Generally accepted
The higher the surface temperature, the faster any loose flour on it scorches rather than simply toasting.

Equipment-specific causes

The same symptom has different answers on different equipment. Each branch below states how to tell whether it is yours.

Workflow and timing

Peels and launchingGenerally accepted
How you would know: There is a layer of black, bitter-tasting powder on the underside of the base and on the baking surface.
Too much dusting flour was carried from the peel onto the hot surface. Loose flour has no moisture to protect it and carbonises on contact at pizza temperatures.
Do now: Brush the surface clean before the next pizza; the burnt flour will otherwise transfer to it as well.
Change next time: Dust the peel lightly and evenly, use semolina rather than flour — coarser grains roll rather than sticking and burn less readily — and consider a perforated peel that sheds excess as you launch.

Rescue it now

Not fixable once baked; brush off any loose burnt residue visible on the base before serving

Prevent it next time

Use only as much bench flour or semolina as needed to prevent sticking, and gently shake or brush off any visible excess from the dough's underside before launching

Launching (with a peel)Pizza steel Pizza stone Semolina rimacinata (finely re-milled durum semolina)

Go deeper

Guides that treat this as an outcome to improve, an environment to work with, or a flavour to understand, rather than only a problem to fix.

Cooking on a pizza stone Peels and launching