Butane blowtorch condensing liquid on heating surface
This is a guess - but I think it might be water formed as a byproduct of butane combustion precipitating on the plate:

$$
\ce{2C4H10(g) + 13O2(g) -&gt; 8CO2(g) + 10H2O(g)}
$$

Aluminum has very high thermal conductivity, so it is possible that the plate is rapidly absorbing heat from the water vapor, causing it to condense. If this stops happening right around the point when the aluminum temperature reaches 100 C, that would be evidence to support this hypothesis.