Preheat the oven to 350°F (175°C). Grease a standard loaf pan with butter or non-stick spray, then line with parchment paper for easy removal.
In a mixing bowl, beat the softened butter and sugar together with a hand or stand mixer on medium speed until the mixture is pale, fluffy, and slightly increased in volume, about 3-5 minutes. Look for a light, creamy texture with visible aeration.
Add eggs one at a time, beating well after each addition to combine fully. The mixture should become smooth and glossy, with no visible separation.
Gradually add the sifted flour, mixing on low speed or folding in by hand, alternating with the milk. Begin and end with the flour. The batter should be thick but pourable; scrape down the sides as needed.
Gently fold the mashed strawberries and vanilla extract into the batter with a spatula, just until evenly incorporated. The strawberries should be visibly streaked throughout the batter, giving it a marbled appearance.
Pour the batter into the prepared loaf pan, smoothing the top with a spatula. Bake in the preheated oven for about 60 minutes, or until a toothpick inserted into the center comes out clean and the top is golden brown. The cake should feel firm to touch and spring back slightly when pressed.
Remove the cake from the oven and allow it to cool in the pan for 15 minutes. Then, transfer it to a wire cooling rack and let it cool completely before slicing. The finished loaf will have a dense crumb speckled with berry bits and a fragrant, baked aroma.
