Make cake:
Preheat oven to 325°F with rack in middle.
Invert bottom of spring form pan and lock on side. Brush pan with melted butter, then chill 2 minutes to set. Line bottom of pan with a round of parchment paper, then brush pan and parchment with another layer of melted butter and chill 2 minutes more. Dust with flour, knocking out excess.
Beat together yolks and 1/2 cup sugar in a large bowl with an electric mixer at high speed until pale and thick, about 3 minutes. At medium speed, beat in oil and lemon zest and juice until just combined. Sift in flour and mix at low speed until just combined.
Beat whites with salt in another large bowl with cleaned beaters at medium-high speed until foamy, then add remaining 1/4 cup sugar a little at a time, beating, and continue to beat until whites just hold soft peaks. Gently fold one third of whites into yolk mixture to lighten, then fold in remaining whites gently but thoroughly. Transfer batter to springform pan, smoothing top, and gently rap against counter once or twice to eliminate any air bubbles. Bake until golden brown (top will crack slightly) and a wooden pick inserted in center of cake comes out clean, 40 to 50 minutes. Cool in pan on a rack 10 minutes, then remove side of pan and cool cake to room temperature, about 1 hour (sides will cave in a little).
Make filling while cake cools:
Whisk together sugar, flour, and salt in a small heavy saucepan, then add lemon juice in a slow stream, whisking until combined. Bring to a boil, whisking constantly, then simmer, whisking, until thickened, about 3 minutes. Remove from heat.
Whisk yolk in a small bowl, then add about one fourth of lemon-juice mixture, whisking vigorously. Whisk into remaining lemon-juice mixture and gently boil, whisking, 1 minute. Remove from heat and stir in butter and zest. Transfer filling to a bowl and cover surface with buttered parchment paper. Chill until cool, at least 30 minutes.
Make filling while cake cools:
Bring cream, honey, and lavender blossoms just to a boil in a small saucepan, then remove from heat and let steep, covered, 30 minutes. Strain through a fine-mesh sieve into a bowl, discarding solids, and chill lavender cream, covered, until cold.
Assemble cake:
Invert cake and discard parchment. Cut cake horizontally into 3 even layers with a long serrated knife. Transfer 1 layer, cut side up, to a cake plate. Lightly whisk filling to loosen, then spread half of filling evenly over cake layer on plate, leaving a 1/2-inch border around edge. Place a second cake layer over filling and spread with remaining filling in same manner. Top with remaining cake layer, cut side down, pressing gently so that filling is spread to edge of cake.
Just before serving, beat lavender cream with a whisk until it is thickened and barely holds soft peaks. Dust cake with confectioners sugar and serve with lavender cream.
