Pulse freeze-dried strawberries in a coffee grinder or food processor to a fine powder. Pass through a fine mesh strainer. Measure out 1/2 cup for the cake batter and 6 Tbsp for the frosting (more if desired). Set aside.
Preheat oven to 350F. Grease and flour three 6" cake rounds and line with parchment.
In a medium bowl, whisk flour, 1/2 cup strawberry powder, baking powder, and salt. Sift to ensure it's well combined. Set aside.
Using a stand mixer fitted with a paddle attachment, cream butter and sugar 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 and milk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition.
Bake for 35-40 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.
Strawberry Mascarpone Buttercream:
Beat butter until smooth. Add mascarpone cheese and beat until smooth and combined.
Slowly add in powdered sugar (1/2 cup at a time). Add vanilla and 1 Tbsp cream and beat on med-high for 5mins. Reduce speed to low and beat for 2mins. Add more cream if desired.
Strawberry powder to be added during assembly below.
Place one layer of cake on a cake stand or serving plate. Top with approximately 1 cup of buttercream, spread evenly. Top with a thin layer of sliced strawberries.**
Repeat with remaining layer and apply a thin layer of frosting all over to crumb coat the cake. Chill for 20mins.
While cake is chilling, add 1/4 cup strawberry powder to the remaining frosting. Beat to combine.***
Frost the top and sides of the cake and smooth with an icing scraper. Using a 1M tip, pipe dollop ruffles onto the top of the cake. If desired, sprinkle top with chopped freeze-dried strawberries and garnish with thyme sprigs.
* You can purchase freeze-dried strawberry powder instead, if it's easier to find. ** The thickness or amount is up to you, but note that adding too much can make the cake a bit unstable and hard to stack -- try to press the berries into the frosting. *** I also added in the strawberry powder from the bottom of the freeze-dried strawberry container as I wanted some seeds throughout the frosting.