Step 1Heat the oven to 350 degrees.
Step 2Beat the butter and sugar in a bowl on medium speed until creamy, 3 to 4 minutes. Mix the flour and baking powder together and beat into the butter mixture.
Step 3Pat the dough into the bottom of a 13x9-inch baking pan. Bake until golden brown, 20 to 25 minutes. Remove from the oven. Let stand until cool.
Step 1Combine the butter, brown sugar, condensed milk and corn syrup in a saucepan and mix well. Bring to a boil over medium heat. Boil, stirring constantly, until a candy thermometer registers 238 degrees, 10 to 12 minutes. Pour over the shortbread layer. Cool.
Step 1Toast the pecans in a dry skillet over medium heat, stirring, until lightly browned, 5 to 6 minutes.
Step 2Heat the chocolate chips in a double boiler set over, but not touching, simmering water or in a microwave, until melted. Stir to blend. Spread the chocolate over the cooled caramel layer. Sprinkle with the pecans and press them lightly into the chocolate. Let cool before cutting into bars.