Let the original cost of each puzzle be \( x \). Since Molly has 6 grandchildren, she buys 6 puzzles.
The total cost of the puzzles, after the discount, can be expressed as:
\[ 6(x - 0.50) \]
Molly spent a total of $27, so we can set up the equation:
\[ 6(x - 0.50) = 27 \]
Now, we can solve for \( x \):
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Distribute the 6: \[ 6x - 3 = 27 \]
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Add 3 to both sides: \[ 6x = 30 \]
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Divide by 6: \[ x = 5 \]
Thus, the original cost of each puzzle is \( \boxed{5} \) dollars.