My maternal grandmother made the most amazing, runny butter tarts, and I was lucky enough to get a copy of her recipe before she passed away ten years ago. I've actually had to share it with a few of my relatives because they'd lost their copies. This year my dad substituted maple syrup for the corn syrup. The tarts ended up a bit less runny, but super tasty. I'm usually lazy and use frozen tart shells, but I forgot them at home this year (we bake at my parents'), so I actually made pastry for them! Very indulgent of my father and husband (as they are the primary eaters, since I can't have dairy this year).
1/3 c butter
3/4 c brown sugar (packed)
1/4 c corn syrup (or maple syrup)
1 tsp vanilla
nuts, raisins or both (or none - my choice!)
Add butter and sugar and beat.
Add other stuff (except nuts & raisins).
Grandma plumped up raisins in hot water first (or you could use rum). She'd put nuts/raisins in tart shells and spooned the wet mixture on top. For yummiest results you want the wet mixture so it would just fill the tart shell, but kind of rounded. This will help avoid it boiling over the edge while cooking.
Bake at 375 for 8 minutes.
Lower temp to 350 until brown (for another 5 to 10 minutes) or done.
I discovered that my old oven doesn't actually get even close to as warm as it claims, so if you like them runny, cook at 325 for 8 minutes then lower to 300 for about 15-20. Thanks to that oven, I can't cook a "solid" butter tart! (Which rocks for my Dad who loves them runny, but Ryan doesn't like them quite that messy.)