Make the cake
In a medium bowl, whisk together the flour, baking powder, baking soda, xanthan gum, and salt until well combined. Stir in the poppy seeds. Set aside.
In a large bowl, using an electric mixer on medium speed, cream the butter and granulated sugar until light and fluffy, about 3 minutes. Scrape down the sides of the bowl. Add the eggs, one at a time, beating well after each addition. Scrape down the sides of the bowl. Stir in the almond and vanilla extracts.
Alternately add the flour mixture and buttermilk to the butter-sugar mixture, starting and ending with the flour mixture, stirring between each addition until blended. Mix until smooth, about 30 seconds.
Spoon the batter into the prepared pan and spread smooth with lightly greased fingers or the back of a greased spoon.
Bake until golden brown and a wooden pick inserted in the center comes out clean, 45 to 55 minutes. Let the cake cool in the pan on a wire rack for 5 minutes. If necessary, gently run a knife blade between the cake and the edge of the pan to loosen. Invert the coffee cake onto a wire rack to cool.
Make the icing
In a medium bowl, combine the confectioners’ sugar, cream, butter, and almond extract. Beat until smooth and spreadable. Spread or drizzle the icing over the cake. Sprinkle the almonds over the top of the icing. Let the icing harden for about 15 minutes before serving the cake.