The filling for butter tarts contains a lot of sugar and sugar is sucrose, which wants to crystallize. If you find your filling is too crystallized, try adding in a little corn syrup (just a tablespoon even) or add a teaspoon of vinegar or lemon juice. The acidity will help break down some of the sucrose into glucose and fructose, and these invert sugars will help keep the filling more fluid. Another suggestion: don't over-bake the filling. I prefer to pre-bake the crusts empty, then fill and briefly bake the final tarts to avoid the filling boiling, which helps keep the filling runny.