Preheat oven to 350F. Grease and flour two 8" cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, poppy seeds, and salt. Set aside.
Using a stand mixer fitted with a paddle attachment, cream butter, sugar and orange zest on med-high until pale and fluffy (about 3mins). Reduce speed and add eggs one at a time fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture and milk & orange juice, beginning and ending with flour (3 additions of flour and 2 of milk & orange juice). Fully incorporating after each addition.
Bake for 25-30 mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack to cool completely.
Orange Simple Syrup:
Place sugar and orange juice into a pot over med-high heat. Stir to dissolve sugar and cook until mixture boils. Boil for 1 minute. Cool completely before use.
Beat butter until smooth. Slowly add in powdered sugar (1/2 cup at a time). Add vanilla and beat on med-high for 5mins.
Add mascarpone cheese and beat just until smooth and combined. Do not overbeat.
Place one layer of cake on a cake stand or serving plate. Brush with 2-3 Tbsp simple syrup. Top with approximately 1 cup of buttercream, spread evenly.
Repeat with remaining layer and apply a thin layer of frosting all over to crumb coat the cake. Chill for 20mins. You can leave the cake naked or fully frost the cake after chilling if desired.
Decorate the top with fresh orange slices, orange leaves, and poppy seeds if desired.