Crush the freeze-dried strawberries to powder using a food processor, blender, or coffee grinder. If you don’t have any of those on hand, you can put them in a ziplock bag and use a rolling pin to crush them as fine as possible.
Preheat the oven to 350 degrees Fahrenheit. Spray a 9 x 13 x 2-inch cake pan with nonstick cooking spray and, if you would like to remove the cake from the pan before serving, line it with a sheet of parchment paper, leaving a 2-inch overhang on either side. Spray the parchment lightly with nonstick cooking spray.
Add 3 cups all-purpose flour, 2 teaspoons baking powder, 1 teaspoon baking soda, and ½ teaspoon salt to a medium bowl and whisk together. Set aside.
Add ½ cup softened unsalted butter and 1 ½ cups granulated sugar to a large bowl and beat on medium-high speed with a hand mixer or a stand mixer fitted with the paddle attachment until fluffy, about 3 minutes.
Add 2 large eggs, 1 ½ cups sour cream, and 2 teaspoons vanilla extract and mix until combined. Add 1 cup powdered freeze-dried strawberries and mix until combined, scraping down the sides and bottom of the bowl as needed.
Add the flour mixture and mix just until combined. The batter will be quite thick. Stir in ¾ cup diced fresh strawberries by hand until evenly distributed in the batter.
Add the batter to the prepared pan and spread evenly with a spatula.
Bake for 30-35 minutes, until a toothpick comes out clean and the center of the cake springs back when lightly pressed on. Remove the cake from the oven and let it cool completely.
Strawberry Buttercream
Crush the freeze-dried strawberries as directed above.
Add 1 cup softened salted butter to a large bowl and beat on high speed with a hand mixer or a stand mixer fitted with the paddle attachment until light and fluffy.
Add 2 ½ cups powdered sugar and 3 tablespoons powdered freeze-dried strawberries and beat on low speed until combined.
Increase the speed and beat until creamy, scraping down the sides and bottom of the bowl as needed. Add 2 tablespoons heavy whipping cream and mix until smooth.
Spread the buttercream evenly over the top of the cooled cake. Garnish with fresh sliced strawberries, if desired, and serve.