Preheat the oven to 350°F. Line an 8-x-8inch baking dish with foil so that it extends up the sides.

Pulse 1/2 cup of the oats in a food processor until finely ground and powdery. Combine in a large bowl with the remaining 2 cups of oats and the baking soda.

Melt the butter in a small saucepan. Add the maple syrup, sugar, and peanut butter and stir until smooth and heated through. Remove from the heat and stir in the maple or vanilla extract and the salt.

Pour the mixture over the oats and stir until well combined. Transfer to the prepared pan and, using a small piece of parchment paper, press into a firm, even layer. Bake until the bars are fragrant and just beginning to turn golden brown, 20 to 25 minutes. Cool for 20 minutes, then lift out in one piece, using the foil as handles. Cut into 9 squares. Let the bars cool completely.

Melt the chocolate in the microwave or in a small bowl set over a pan of simmering water. Let cool for 5 minutes. Spoon the chocolate into a small sandwich bag and snip off one corner. Drizzle the chocolate over the bars. Let stand until the chocolate is set, about 1 hour.