Preheat oven to 350 degrees. Spray bundt cake pan with baking spray.
In a large mixing bowl, beat almond paste and sugar until the mixture resembles cornmeal. Add eggs, one at a time, mixing well after each addition. Add vanilla and butter; beat until smooth.
Add flour, baking powder, and salt to the almond mixture, alternating with the milk. Stir in poppy seeds.
Pour mixture into prepared bundt cake pan and spread evenly with back of wooden spoon. Bake until a cake tester or tooth pick inserted in center comes out clean, about 50-55 minutes. Let cake cool in pan for about 10-15 minutes, then remove from pan to wire rack. Cool completely. Dust with confectioner's sugar as desired.