Preheat the oven to 375° and line a 10-by-15-inch jelly roll pan with parchment paper. Spread the pecans in a pie plate and toast for about 8 minutes, until fragrant. Let cool, then coarsely chop the pecans.
In a small bowl, whisk the flour with the baking powder, baking soda and salt. In a medium bowl, using an electric mixer, beat the butter with the brown sugar at medium-high speed until smooth. Beat in the egg and vanilla. At low speed, beat in the dry ingredients. Spread the dough onto the entire baking sheet in an even layer and sprinkle the chopped pecans on top. Freeze for 5 minutes, just until firm.
Bake the bar in the center of the oven for about 12 minutes, until golden. Let cool completely, then invert the bar onto a work surface. Peel off the paper and cut the bar into 2 equal pieces. Spread the softened ice cream on one half and top with the other half, nut side up. Return to the freezer and freeze until firm, at least 1 hour. Cut into 8 bars and serve right away.