Lightly butter a metal 8x8 baking pan and line it with parchment paper on all sides. Preheat the oven to 350 F/180 C.
Mix the melted butter, flour, sugar, brown sugar, and vanilla paste. Keep in the fridge until ready to use.
Toss the chopped strawberries, sugar, flour, and lemon juice until well coated. Set aside.
In a small bowl, whisk together the flour, baking powder, and salt and set aside.
In a mixing bowl, cream together the butter and sugar until light and fluffy, about 5 minutes.
Scrape down the bowl. Slowly stream in the whisked eggs and vanilla, mixing very well and scraping down the bowl frequently.
Add in half of the dry ingredients and mix on low until mostly combined. Then mix in the buttermilk, followed by the remainder of the dry ingredients, mixing until just combined.
Spread the vanilla cake batter evenly into the prepared baking pan, then spread the strawberries evenly over the top. Break up the crumb topping and sprinkle it evenly across the strawberries. Bake in the preheated oven for 55-60 minutes, or until a toothpick inserted into the center comes out clean.
Let the pan cool on a wire rack for 20 minutes, then remove it from the pan and let it cool completely before cutting. Dust with powdered sugar if desired and enjoy!