Grease an 8x8x2-inch baking pan. Preheat oven to 350 F
Beat 6 T softened butter in a medium mixing bowl with an electric mixer on medium to high speed for 30 seconds. Add 1/4 cup sugar and beat until combined. Beat in 1 cup of the flour and xantham gum (mixture will be crumbly). Press the mixture into the prepared pan. Bake in a 350 F oven for 10 minutes.
Combine 1/4 cup flour and 2 tablespoons sugar in a small mixing bowl or food processor. Cut in the almond paste and the 2 tablespoons butter with a pastry blender until crumbly.
Spread jam over top of the hot baked crust. Sprinkle almond paste mixture over the jam. Sprinkle with sliced almonds. Bake in the 350 degree F oven for 20 to 25 minutes. Cool in the pan on a wire rack.
For icing, stir together powdered sugar and 1 teaspoon milk. Add additional milk, 1/4 teaspoon at a time, until icing is easy to drizzle. Drizzle over the top and then cut into squares. Makes 16 squares.