Preheat the oven to 350°F (180°C). Butter a 9- by 5-inch (23- by 13-cm) or 8 1/2- by 4 1/2-inch (22- by 12-cm) loaf pan and line it with parchment paper.
In a small skillet over medium heat, dry roast the sesame seeds until fragrant, 2 to 4 minutes. Transfer to a mortar and crush them lightly with the pestle.
In a large bowl, combine the butter, sugar, flour, baking powder, eggs, zest, and juice, and mix with a hand mixer until the batter is light and creamy, about 2 minutes. Take care not to overbeat the batter as this can result in your cake sinking during baking. Add the crushed sesame seeds.
Pour the batter into the prepared pan and bake until a skewer inserted into the center of the cake comes out clean, 35 to 40 minutes. Leave the cake to cool in the pan for 10 minutes, then turn out onto a wire rack and leave to cool completely.