Preheat your oven to 350°F (175°C). Lightly butter a medium cake pan (around 8x8-inch).
In the bottom of the buttered pan, layer 1 1/2 cups of frozen berries. Drizzle the melted butter over the berries, then sprinkle with brown sugar and a touch of nutmeg.
In a large mixing bowl, cream together the softened butter and sugar until light and fluffy. Add in the eggs one at a time, beating after each. Mix in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, and salt. Add dry ingredients to the wet, alternating with milk, and mix until a smooth batter forms.
Pour the vanilla cake batter gently over the berry mixture in the pan, spreading evenly.
Top the batter with the remaining 1 1/2 cups of frozen berries.
Bake in the preheated oven for 40 minutes, or until a toothpick inserted in the center comes out clean and the top is golden.
Let cool slightly before serving. Delicious warm or cold, plain or topped with whipped cream or vanilla ice cream.