Preheat oven to 350°; place an oven rack in lower third of oven.
Spray an 8-inch cake pan with a gluten-free nonstick coating. If using, spread almonds and sugar evenly over bottom of pan. It’s best to select the prettiest almond pieces, because they will show when the cake is turned out. If not using nuts, place an 8-inch parchment round in cake pan and spray with gluten-free nonstick coating.
In a small bowl, whisk together almond flour, oat flour, xanthan gum, baking powder and salt.
In bowl of a stand mixer fitted with paddle attachment or in a large bowl and using a hand mixer, combine granulated sugar, butter and orange zest, and beat on medium speed. Add almond extract, amaretto and eggs, one at a time; mix on low speed after each addition, until batter is fluffy and lighter in color.
Add dry ingredients in two batches, mixing on low speed after each addition until all ingredients are well incorporated, about 1 minute.
Using a rubber spatula, transfer batter to prepared pan and smooth the top. Bake 35–40 minutes, until top is golden brown, and a toothpick inserted in the middle comes out clean.
Remove pan from oven. Let cake cool 10 minutes; then turn it out onto a cake plate or cake stand. Dust with confectioners’ sugar and serve.