Grease and flour a 10-inch (3 L) tube pan. (I often use a bunt pan / ring mould instead).
Sift together: the flour, baking powder, baking soda, and salt.
In a large bowl, beat the eggs with an electric mixer until thick and light-coloured. Gradually add the sugar, 1/4 cup at a time, beating well after each addition. Stir in the oil and almond extract. Blend in the dry ingredients, mixing until smooth. Stir in the zucchini and ground almonds.
Pour the batter into the prepared pan and bake in a 350 F (180 C) oven for 1 hour and 15 minutes, or until a tester inserted into the centre comes out clean.
Cool the cake in the pan on a rack for 15 minutes. Remove the cake from the pan and cool it completely on a rack.
For the glaze, combine the icing sugar, milk, and almond extract together in a small bowl, stirring until smooth. Spread over the top of the cooled cake, letting the icing run down the sides. Sprinkle the sliced almonds overtop.