Preheat oven to 350F (180C). Using electric mixer, beat butter and sugar until light and fluffy. Beat in vanilla.

In separate bowl, combine flour, rice flour, baking powder and salt; using wooden spoon, gradually stir flour mixture into butter mixture, stirring well, until dough holds together.

Press dough evenly onto bottom of parchment paper lined 9" (2.5L) baking pan. Bake in centre of oven for about 20 minutes or until light golden.

Brownies: Melt butter and chocolate together. Whisk in sugar. Whisk in egg and vanilla. Stir in flour. Spread evenly over shortbread layer. Bake for 20 minutes or until tester inserted in centre comes out with moist crumbs. Cool in pan on rack. Drizzle melted white chocolate over top.  (Brownies can be stored in airtight container for up to 5 days).

Makes 24 pieces.