Preheat oven to 400 degrees. In a mixing bowl, whisk together flour, baking powder, baking soda, salt and poppy seeds, set aside. In the bowl of an electric stand mixer, attached with paddle attachment, blend together butter and sugar on medium speed until pale and fluffy.
Add in egg, egg yolk and almond extract and mix until blended. Measure out milk into a liquid measuring cup then whisk sour cream into milk. Working in three separate batches beginning and ending with flour mixture, add flour mixture alternating with milk mixture to batter and mix on low speed just until combine after each addition.
Divide batter among 12 paper lined muffin cups (in a non-stick muffin tin), sprinkle slivered almonds evenly among muffins and bake in preheated oven 16 - 18 minutes until lightly golden and toothpick inserted into the center comes out clean. Allow to cool and store in an airtight container.