Preheat the oven to 350ºF.
Line an 8-inch square baking pan with parchment paper, allowing it to hang over two sides. Coat the paper with cooking spray.
In a stand mixer fitted with the whisk attachment, whisk together the melted butter with the sugar and light brown sugar until smooth, about 2 minutes. Add the eggs and vanilla and whisk until combined.
Remove the bowl from the stand mixer and stir in the flour and salt just until incorporated. Stir in 1 cup of the chocolate chips and 1/2 a cup of the crushed pretzels.
Pour the batter into the prepared pan, smoothing the top, then sprinkle the remaining 1/2 cup of crushed pretzels on top, pressing them lightly into the batter.
Bake for about 30 until a toothpick inserted comes out clean. While the cookie bars are baking, melt the remaining 1/2 cup chocolate chips with the vegetable oil in a double boiler or in the microwave.
Remove the cookie bars from the oven and allow them to cool completely in the pan then remove the cookies using the parchment paper overhang.
Drizzle the cookie bars with the melted chocolate and allow them to cool until the chocolate hardens. Cut the cookie bars into 16 squares and serve.