Lightly grease the inside of a 4-quart saucepan.

Line a baking sheet with waxed paper.

Stir the brown sugar, white sugar, baking soda, sour cream, and corn syrup together in the oiled pan; place over low heat.

Cook and stir continually until the sugars are dissolved.

Wipe any sugar crystals above the liquid line with a pastry brush moistened with cold water.

Bring to a boil over medium-high heat.

Do not stir once the sugar syrup begins to boil.

Heat to 240 degrees F (115 degrees C), or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface.

Remove from heat; stir in the butter, vanilla, and chopped pecans.

Drop spoonfuls of the mixture 2 inches apart onto the prepared baking sheets.

Press a pecan half into the top of each praline.

Allow to cool completely.

Store in an airtight container in a cool place.