Prepare the pan. Preheat your oven to 350°F (180°C). Grease a 9x5-inch loaf pan generously with the 1 tablespoon of butter set aside for this purpose.
Cream the butter and sugar. In the bowl of a mixer fitted with the paddle attachment, beat the remaining 2 sticks of butter with the sugar on high speed until pale, light, and fluffy — about 5 to 8 minutes.
Add the eggs. Beat in the eggs one at a time, mixing well after each one so the batter stays smooth.
Add the condensed milk and vanilla. In a small bowl, whisk together the ¾ cup condensed milk and the vanilla extract. Pour this into the mixer bowl and beat until fully combined.
Fold in the dry ingredients. Remove the bowl from the mixer. Using a spatula, gently fold in the flour, baking powder, and salt by hand until just combined — do not overmix. Transfer the batter to the prepared loaf pan and smooth the top.
Bake. Bake for about 1 hour, or until a skewer inserted into the centre comes out clean.
Cool. Let the cake rest in the pan for 15 minutes, then invert it onto a serving platter and allow it to cool completely.
Make the glaze. In a medium saucepan, bring the ½ cup condensed milk, red-wine vinegar, and the lime zest and juice to a simmer. Cook for 1 minute, then remove from the heat.
Glaze the cake. Pour the warm glaze evenly over the fully cooled cake, letting it settle into the top and drip down the sides. Slice and serve.
