Let ice cream melt at room temperature.
Heat oven to 350°F. Coat 15 x 10 x 1-inch pan with nonstick spray. Line with foil, extending over sides; coat foil.
In medium bowl, whisk flour, salt, baking powder, allspice and nutmeg. In large bowl, beat together butter and sugar until smooth. Beat in melted ice cream, scraping down bowl. At low speed, beat in flour mixture until smooth. Fold in 1 cup butterscotch chips. Spread batter in prepared pan.
Bake in 350°F oven until dry to touch, about 25 minutes. Let cool in pan on wire rack.
Smoothly spread bar with frosting. Melt remaining 1/4 cup chips with oil in glass bowl in microwave oven on high for 45 seconds. Stir until smooth. Pipe in lines over frosted bar; lightly drag knife across lines. Refrigerate.