Preheat oven to 350F. Grease and flour three 6" cake rounds and line with parchment.
In a medium bowl, whisk flour, poppy seeds, baking powder, and salt until well combined. Set aside.
Using a stand mixer fitted with a paddle attachment, cream butter and lemon zest until smooth. Add sugar and beat on med-high until pale and fluffy (approx 3mins).
Reduce speed and add eggs one at a time fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture with milk & lemon juice, beginning and ending with flour (3 additions of flour and 2 of milk & lemon juice). Fully incorporating after each addition.
Spread batter evenly between the prepared pans and smooth the tops.
Bake for about 30-35mins 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.
Lemon Simple Syrup:
Place lemon juice and sugar into a small saucepan, stir to combine. Cook over medium heat, stirring often, until simmering and all sugar has dissolved. Cool completely before using on cake.
Cream Cheese Frosting:
Using a stand mixer or hand mixer beat butter until pale and creamy.
Reduce speed to low. Add powdered sugar 1 cup at a time and mix on low until well blended. Increase speed to medium and beat for 3 minutes. Add vanilla and continue to beat on medium for 1 minute.
Slowly add chilled cream cheese one cube at a time (about 1 tablespoon at a time). Beat well until blended and ensure no lumps of cream cheese remain.
If needed, add cornstarch or meringue powder 1 tablespoon at a time to stiffen the frosting (I didn't but this is an option). Add a pinch of salt to cut some sweetness if desired.
Run mixer on low for a couple of minutes to remove any air bubbles.
Assembly:
Trim the crust off the top of each cake layer and brush each with the lemon simple syrup. About 1-2 Tbsp per layer.
Place one layer of cake on a cake stand or serving plate. Top with 2/3 cup of frosting, spread evenly. Repeat with the remaining layers and apply a thin coat of frosting all over the cake. Chill for 20mins.
Use the remaining frosting to frost the cake in a rustic manner.
Decorate with fresh, edible flowers and lemon slices if desired.