Preheat oven to 450°F (230°C).
Pour the melted salted butter into a 9x9-inch baking dish. Tilt the pan to evenly coat the bottom. Set aside.
In a large bowl, whisk together the flour, baking powder, salt, and sugar.
In a medium bowl, whisk together the buttermilk, melted butter, and vanilla extract.
Pour the wet ingredients into the dry ingredients. Stir gently until just combined. Do not overmix. The batter should be thick and slightly lumpy.
Gently fold in the diced strawberries until evenly distributed.
Pour the batter into the prepared baking dish and spread evenly. Lightly score into 9 squares with a knife.
Bake for 25–30 minutes, or until golden on top and set in the center.
Remove from the oven and let cool for 10–15 minutes.
In a medium bowl, whisk together the powdered sugar, cream or milk, vanilla, and salt until smooth.
Stir in the diced strawberries and strawberry jam (if using).
Adjust consistency as needed with more milk or powdered sugar.
Drizzle the glaze over the warm (not hot) biscuits.
Let set for 5–10 minutes before slicing and serving.
