Combine butter and chocolate in a large, heavy saucepan over low heat. Cook, stirring occasionally, just until butter and chocolate melt. Remove from heat.
Stir in brown sugar and next 3 ingredients.
Combine flour, baking powder, and salt. Stir into chocolate mixture. Stir in walnuts.
Pour batter into a foil-lined 13- x 9-inch baking dish. Bake at 350° for 30 minutes. Cool and cut into squares. Top with ice cream, Chocolate-Walnut Ganache, strawberries, and whipped cream.