Preheat oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving overhang for easy removal. Puree the 1 cup strawberries (plus optional sugar) until completely smooth. Set aside.
In a large microwave-safe bowl, melt the butter. Whisk in the 1½ cups sugar until smooth and slightly glossy. Add eggs one at a time, whisking well after each. Stir in vanilla.
Sift in cocoa powder, flour, and salt. Stir gently until just combined—do not overmix. Fold in the strawberry puree until evenly distributed. The batter will look thick and slightly pink-tinged.
Pour batter into the prepared pan and smooth the top. Bake 30–35 minutes. The edges should look set and slightly pulled away from the pan. A toothpick inserted in the center should come out with a few moist crumbs, not wet batter.
Let brownies cool in the pan for 20 minutes, then lift out using the parchment and cool fully on a wire rack before slicing. This sets the fudgy texture.
Once cool, arrange sliced strawberries on top and drizzle with melted white chocolate if using. Slice into 16 squares.