Remove all unpopped kernels from popped corn. Place popcorn into 17x12x12 inch baking pan. In a 1½ quart heavy bottomed saucepan, combine sugar, butter and syrup. Cook and stir over medium heat till butter melts and mixture comes to boiling. Cook, stirring occasionally, to 255 degrees (hard ball stage), about 4 minutes more. Remove from heat. Add mixture of soda and vanilla. Pour over popcorn; stir to coat. Bake in 300 degree oven for 15 minutes, stir. Bake another 5 minutes. Transfer to waxed paper on counter top. Cool
Recipe by The Gingham Apron at https://www.theginghamapron.com/kettle-corn/