Preparation

• Position a rack in the center of the oven and heat to 350°F. Line an 8-inch round cake pan with parchment. Lightly butter the parchment and the side of the pan.
• Evenly spread the streusel in the prepared pan. Transfer to the oven and bake for 10 minutes. Set aside.
• In a small saucepan, melt the butter over low heat and keep warm.
• Fill a medium saucepan about one-third full of water and heat to just under
  a simmer.
• In the bowl of a stand mixer, combine the eggs, sugar, and zest. Place the bowl over the simmering water, and whisk until the sugar dissolves, about 2 minutes. Fit the mixer with the whisk attachment, then whip the egg mixture on medium high until thick, pale yellow, and double in volume, about 3 minutes.
• In a small bowl, whisk the olive oil, milk, and lemon juice. In a separate small bowl, whisk the flour, baking powder, and salt.
• With the mixer on medium, drizzle the olive oil mixture into the eggs. Turn the mixer to low and add the dry ingredients, stopping as soon as they are incorporated and scraping the side of the bowl as necessary. Fold in the melted butter.
• Pour the batter over the streusel, spreading it to cover evenly. The batter will barely cover the streusel.
• Bake, rotating the pan halfway through, until the cake is golden and bounces back
  to the touch in the center, 38 to 40 minutes.
• Let the cake cool in the pan on a wire rack for about 20 minutes. Run a knife around the edge, then invert the cake onto a serving plate. Dust the cake with confectioners’ sugar, if you like. Serve warm or at room temperature.