Spray the bottom of an 8-by-8-inch baking pan with cooking spray. Line the baking pan with overlapping pieces of parchment, leaving enough to hang over the sides of the pan; this will help you remove the fudge from the pan. Spray the pieces of parchment with cooking spray.
Combine the corn syrup, sugar, whole milk, cocoa powder and butter in a heavy-bottom saucepan fitted with a candy thermometer over medium heat. Stir until the sugar and butter melt. Stop stirring and let the mixture continue to cook until the thermometer reaches 238 degrees F, 7 to 10 minutes. Once the mixture reaches 238 degrees F, stir in the vanilla extract and turn off the heat immediately. Pour the spoon fudge into the prepared 8-by-8-inch baking pan, tapping gently so the fudge is evenly distributed. Cool to room temperature, and then place in the refrigerator to set until firm enough to cut, about 1 hour.
Once the fudge is set, use the parchment to lift the candy out of the baking pan onto a cutting board. Peel off the parchment. Cut the fudge into 1-by-2-inch pieces and wrap each with waxed paper. Store in the refrigerator until ready to serve or up to 1 week.
Homemade Vanilla Extract:
Yield:8 to 12 ounces extract
Split the vanilla beans lengthwise, leaving them attached at one end. Place them in a 10- to 12-ounce bottle and cover with the liquor. Put on the top and shake vigorously. Store at room temperature for 4 weeks to let the flavor develop and mature, shaking the bottle once a week. The extract will keep at room temperature in a dark place (like your cabinet) for up to 6 months.