In a 3-quart heavy saucepan, combine butter, granulated sugar, brown sugar, cream, baking soda, salt, and corn syrup if using (see note). Cook over medium-high heat, stirring constantly until it reaches a rolling boil, about 5 to 6 minutes.
Stop stirring and insert a candy thermometer in the mixture. Heat until the mixture reaches 235℉-240℉ or “soft ball” stage. The mixture will bubble up quite a bit and be very foamy. Reaching soft ball stage will take about 4 minutes.
Remove from heat and allow the mixture to cool for 5 minutes.
While the mixture is cooling, cover counter surface or 2 large baking sheets with parchment paper.
Stir in vanilla and pecan halves. Continue stirring until the mixture slightly thickens, about 1 to 2 minutes.
After about 1 minute, using a cookie scoop (about 2 tablespoons), drop the nut mixture onto the prepared parchment. If it is pooling and fails to hold shape, it’s not quite ready. (If you don't have a cookie scoop, use 2 spoons: one spoon to scoop, one spoon to push the mixture out.)
Let the pralines cool for at least 15 minutes. Enjoy a warm praline or allow them to cool completely, about 45 minutes.
Notes
Adding corn syrup: To reduce crystallization (white spots), add 1 tablespoon light corn syrup. This is an optional ingredient because minimal crystallization is normal and does not affect the flavor of the pralines.
Storage: Once they are completely cool, place pralines in an airtight container or bag. They'll keep at room temperature for 1 to 2 weeks or in the freezer for up two months.
If you prefer, substitute 2 cups of walnuts or another nut of your choice.