Heat oven to 375 degrees.
Cream butter and peanut butter. Gradually beat in 1/3 cup sugar and brown sugar. Add egg, milk, and vanilla; beat well.
Combine flour, baking soda and salt. Gradually add dry ingredients to creamed mixture. Shape into 1-inch balls. Roll in sugar.
Place on ungreased cookie sheets. Bake for 9 to 11 minutes.
Press 1 chocolate kiss into the center of each cookie and remove from cookie sheet. Let cool and sit for 2 hours so that chocolate sets. Makes approximately 4 dozen cookies.