In a saucepan, melt 1/2 cup butter. Stir in 2 cups brown sugar and 1/2 cup milk. Bring to a boil and stir frequently for 2 minutes.
Remove from heat and stir in 3/4 cup peanut butter and 1 teaspoon vanilla extract.
In a large bowl, add 3 and 1/2 cups confectioner’s sugar. Pour in the peanut butter mixture and use an electric beater to mix until smooth.
Transfer the mixture into an 8-inch square dish.
Place it in the freezer for about an hour. Once firm, slice and serve.