Preheat oven to 350˚F. Line a sheet pan with a large enough piece of foil to tuck edges under rim of pan. Spray foil-lined pan generously with cooking spray. Also spray a 2 quart microwave-safe bowl and a heat resistant spatula with cooking spray. Place pan in the oven. (Don't worry if your oven never gets to 350˚F. You just want the pan to be nice and warm when you add the candy mixture later. The warm pan will help the brittle to spread better and get nice and thin.) Gather ingredients and a teaspoon measuring spoon.
Combine sugar and corn syrup in the prepared, 2 quart glass bowl and stir to combine. Mixture will be very thick and sugary. Place in microwave and cook on high power for 3 ½ minutes. (It will be clear but still pretty light colored.)*
Carefully remove from microwave (it will be hot!) and add pecans. Stir well and return mixture to microwave. Cook on high power for 3 minutes. (At this point mixture will be a fairly light, golden brown color.)*
Remove from microwave and add butter and vanilla. Stir well and microwave one last time for 1 minute. During this last stint in the microwave, wash your spatula or grab a new one and spray it well with cooking spray. (Mixture should now be a medium amber color.)* Remove pan from the oven and set it on the stovetop.
Carefully remove bowl from microwave and sprinkle baking soda over the top. Stir until baking soda is just incorporated. Mixture will foam up and go from an amber color to caramel.
Immediately, pour out mixture onto heated, foil-lined pan, trying to spread out, as you pour, over as much of the pan as possible. With your heat resistant spatula, spread mixture as thin as possible over the surface of the pan. Don’t worry if it tears or gets a hole here or there. It will be broken up later anyways.
Cool completely, lift foil out of pan and you'll be able to break Pecan Brittle into pieces. Store in an airtight container.