How to Make It
Heat oven to 325° F. Line the bottom and sides of a 9-by-13-inch baking pan with aluminum foil; butter the foil.
In a medium bowl, stir together the oats, flour, baking soda, cinnamon, and salt.
In a large bowl, using a wooden spoon, mix together the butter, sugar, and vanilla until creamy. Stir in the oat mixture, raisins, and nuts until combined.
Press the oat-butter mixture evenly into the prepared pan. Bake until the edges are golden, 18 to 20 minutes. While still warm, cut into 15 bars. Let cool completely in the pan.
Meanwhile, in a double boiler or a medium heatproof bowl set over (not in) a saucepan of simmering water, melt the chocolate, stirring often, until smooth. Drizzle over the cooled bars and chill until set, 15 to 20 minutes.
Storage suggestion: Keep the bars at room temperature, individually wrapped or between sheets of wax paper in an airtight container, for up to 5 days.