Article II, Section 1.2 ...
Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress ...
Most people don't know this, but under the U.S. Constitution individual citizens don't even have the right to vote for President.
If a state legislature wanted to appoint their own slate of electors without having any vote on Election Day, they are perfectly free to do so.